Very good lens.  Light, reasonably fast and great image quality
582966
I had the ED version of the 70-300 which liked because its light weight.  I wasn't happy with its focusing speed and maximun aperture but it was very useful for traveling and some wildlife (felt too limited because the lack of speed).

I tried the VR version of the lens and it is great, it is  heavier than the previous version but still light and relatively small to carry when weight is a concern.  The image quality is certainly better, not as good as the 70-200 2.8, but most of the times you don't need that little difference in quality.  The VR makes a difference and makes it a lot more useful for most situations and the focusing speed is noticeably improved.  For my needs it is a very good lens.
3359121
I bought this lens for it's low light capabilities and I was not disappointed. I used it for inside shooting w/out a flash and "freezing" action in good light like hummingbirds. Very "fast" lens. I eventually sold this for it's slightly sturdier and a little faster cousin the Canon 50mm 1.4. Honestly I don't see a huge  difference. This is a good bang for the buck lens.
1086494
I just recently bought this lens when it was on sale.  I have walked around a local lake to take some pictures, more nature pictures rather than portraits for the time being.

Using a x-t1 with high performance mode on:  This lens is pretty fast focusing for Fuji depending on the focus point box size.  On the larger sizes, I was able to grab focus on flying ducks and geese mid air.  I was also able to track runners around the lake.  Both coming toward the camera and running away from the camera.  Both AF-s and AF-c seems to do well.  I think its a little faster than the xf55-200.  On the smaller focus point sizes, it was a tad slower.  It is also a bit slower to focus if I am changing between extremely close and far subjects.

The output is good, which I have come to expect from Fuji.  We all know the pictures will be great, or why else are we even thinking about dropping $800 for a single lens.  I have an outdoor wedding coming up and I can't wait to try it out then.

I have attached some photos from this lens from a trip to the zoo after purchase.  I did a little editing, but they were all shot in jpeg in fuji standard film simulation.

Added 3/23/16
I just had a chance to shoot a portrait session last week.  This lens did exceptionally well.  Very sharp and contrasty.  Bokeh rendering was good.  The focus speed was quick - I was using eye detect focus and the subjects usually shift around between shots.  Although that was fast enough.  The focus was still a touch slow when it comes to focusing between a near and far subject.
3984841
I've been using L series lenses for a couple of years now, ranging from the 17-40 f/4L to the 135mm f/2L. Currently, I own a Canon 5D Mark III, Canon 70D, 24-70mm f/2.8L Mark I, 135mm f/2L, and this 24mm f1.4L II. Primes, zooms, it doesn't matter, they are all pretty darn good. Primes, however, generally have better all around Image Quality than zooms (with close exceptions going to the 24-70mm f/2.8L II and, from what I hear anyway, the 70-200mm f/2.8L II) and this 24mm f/1.4L II is no exception. Wide open at f/1.4 it is superbly sharp. I've owned the 50mm f/1.2L and I sold it for a loss after a month because it was SO soft at f/1.2. I wasn't missing focus (I know this because I also used Live View and it wasn't very good), it just wasn't sharp. This 24 f/1.4 is wonderfully made. It is classic L quality, both inside and out. Focus ring, filter threads, USM motor, it's all sublime. I always have it on one of my cameras and it's always the first one I want to use when I go shoot.

The focus ring is very smooth, and it has a decent amount of throw in it when going from closet focus to infinity focus. Auto focus is very quick and accurate which allows me to just worry about framing or quick shooting. On my 70D, the live view/movie focusing really is amazing. It's fast, accurate, and quiet. If compared to a hand focused video you would be able to tell the difference with the favor going to the auto-focused.

There is one thing that has caught my eye with this lens, and it's something that has bothered me sometimes. When shooting a close to semi close subject with a brighter background, the blown out highlights draw attention to themselves. This goes along the lines of other "subjective" wants and preferences with photography, so it's up to you as to whether or not it's a big deal. The attached picture shows an example of the issue if look you at the grass on the right but it also shows the sharpness if you look the closest eye.

All that being said, even with the (to me anyway) eye distracting Bokeh, I love my 24mm f/1.4L II USM. It's by far my favorite lens out of all of the ones that have crossed my path with the legendary 135mm f/2L USM coming in an equally solid 2nd.
3734822
I've had this lens for a month or so now. I took it to Tokyo and Kyoto on a trip and took nearly 1000 shots with it. I must say, I am thoroughly impressed with this lens. The image quality is superb. I shoot with a Nikon D7100, and it is very sharp corner to corner on a DX body from 24-80mm. Above that to 105, image quality may start to soften slightly, but it's negligible. I've never shot the Canon 24-105 or the Nikon 24-120 to be honest, but after extensive research and reviews, I settled on this over the Nikon 24-120, and I'm very happy with it. The build quality is tops. The lens is a bit on the heavy side, but you're getting quality glass, and I had no issues carrying it around all day with my BlackRapid sling. A couple things to be aware of... first, the lens focuses backward (counterclockwise, like Canon) compared to the normal Nikon clockwise focusing fashion. It was a little weird at first but you get used to it, no biggie at all. Second, there's been reported issues with Sigma lenses and the Nikon D7100 (and maybe other Nikon's as well) concerning image review. With the Sigma lens attached, you cannot hold the arrow button down to pan around an image for review, it requires clicking each time. Also, if you have a screen timeout set for 4 seconds, the image review will stay on for about 1 minute with this lens on.  Once a Nikon lens is put on, your image review functions go back to normal, even when looking at images taken with the Sigma lens.  It's a known issue, and apparently Sigma isn't planning to release a firmware update for it (I have no idea why...). It's a minor nuisance, and in my opinion, the quality of the lens outweighs those cons. I'll be interested to see if the problem persists with my new D610 when it arrives. Regardless, I still think the lens is a very worthy standard zoom that can outclass both the Nikon 24-85 and the 24-120 in both image quality and construction quality, and also the aging Canon 24-105! Do yourself a favor and check one out!
2025882
I am real happy with this lens. It is light weight and the price is right.
Easy to use.
455732
The only problem that i have with this lens is that it's f4.

i wish it was 2.8! Then it would have been an awesome lens.

Auto focus is alright...not slow....not very fast.

The lens is sharp. And it's built well.

I also have 85mm 1.2L, 50mm 1.8 on the side.. But this L lens as my primary general purpose lens.
3639518
This lens has excellent image quality, sharpness and colors. The build quality is not top notch but for this price who cares?
2768343
I love this lens.  A bit heavy for the camera but not to bad.  You know I have a good selection of lens for my XT1's and they do add a little bit of weight to the lightweight mirrorless cameras.  Sometimes I wonder if some of these lens defeats the purpose of the mirrorless cameras.  But I can't say this is a heavy lens.  It works perfectly on my camera body.  Balanced well especially with the metal hand grip I use.  And images are extremely sharp.  This is a beautiful lens.  And focus fast.  For me.  I'm very happy with this lens
2206276
Sharp, beautiful, relatively light-weight. This lens is so, so wonderful at landscape photography. The colors are bold and vivid, the image stabilization works extremely well, and the auto-focus is quick and rarely hunts.
4598604
Great little lens. Bought this based on Ken Rockwell recommendation after I got my new D5300. It's light, compact, sharp as a tack and fast. Only taken a few hundred shots so far with it, but very pleased so far. Have used it for Landscapes, night photos and even indoors at a poorly lit gym for a volleyball match (got some great shots at 1/640).
2480389
I am very impressed by this lens. Its small, lightweight, portable, and extends to 400mm which is amazing for a lens that is so compact. It takes good photos. The only caveat is that there is some chromatic aberration which is not easily eradicated in Photoshop or Lightroom. But the for the size and the quality of the images, its well worth it.
4173501
This is a fast lens that produces sharp image at f2.2 and above. The chromatic aberration is HORRIBLE, however! Yes, it's fixable with either the Photoshop or Lightroom, but that is very unproductive if you have lots of images to correct. But I'd still recommend it for its lightweight and relatively good image quality.
604898
This is the lens I leave on my camera for general purpose use.  At non-macro focus distances, it is a very fast lens in a convenient short-telephoto length.  The images are sharp out to the corners.  This is one of those great Canon lenses that probably deserves an L-series rating but is available without the red stripe and exorbitant cost.

Tested in an astrophotography context (point sources on black backgrounds - excellent for revealing aberration and coma), I have found that this lens is reasonably sharp at f/4 and completely sharp by f/5.6.  For daylight terrestrial photography, the tiny aberrations caused by the wide-open f/2.8 aperture are hardy noticeable and by f/4 the images are exquisitely sharp.

There is a lot of glass in this lens - it is heavy.  I highly recommend buying the tripod collar as it allows you to attach the camera and lens to a tripod or ballhead with much better balance.

Also, if you plan on using the macro functionality, keep in mind that the effective f-stop of the lens goes way up when you're focusing at very close range.  At 15cm from the front surface, it performs as though it were an f/9.  You'll either want a lot of light or a very steady subject and a good tripod (another reason to buy the hideously overpriced tripod collar).
4906889
Super fast (F1.4), very sharp over the entire field, 12mm wide-angle (equivalent of 24mm on full-frame), weather-sealed lens for MFT cameras.  Used it on Olympus PEN EPL-5, -7, and M5 mkii bodies (due to its size it feels more comfortable on M5). Great for landscapes and crowds, street shooting, where low light conditions may prevail.  Also should be great for astrophotography although I have not had a chance to try it yet.

PROS: Nicely finished metal body construction with smooth-as-silk focusing ring in manual mode.  Razor sharp lens even fully open at F1.4.  Fantastically small focusing distance, great for popping objects from the blurred background - very nice and creamy bokeh. Auto focus is very quick (at least on OLY bodies). Color rendition is very realistic and the contrast is good in most lighting conditions. No noticable vignetting. Flare is nicely controlled. Comes with a high quality (metal) lens hood and nice plastic lens cap.

CONS: Heavy and large (this is the physics of F1.4 - you need a lot of glass). Significantly bigger than the Oly F2.0 12mm and Samyang models. ~ 4 pixels of chromatic aberration at edges (purple fringing) under high contrast scenes. Autofocus is audible (not loud per say, but noticable). The aperature ring does NOT work with OLY cameras - not the lens' fault so no points deducted.
A bit expensive as compared with other lenses in this class - included lens bag is disappointing lightweight nylon.

Night shots below were done on an EPL5 with ASA 400 and F1.4.
456393
198 out of 214 Amazon reviews gave this a 4 or 5-star rating. That's a 93% favorable rating. Pretty good for a lens at this price point.

I have it and also have the 70-200 f/2.8 at $1600.

For regular family shots, newspaper shoots, this is a 4-star lens. It is very light, too. VR works very well. The f/2.8 is like carrying aroung a lead pipe, so I leave it home when I shoot parades or have good lighting situations. When I want the best I use the f/2.8...but, you'll find critics of this lens, too.

Let's face it: Most lenses today are pretty darn good. At this price it's hard to pass up. If you don't have a long lens, want to have some fun, and not rupture your bank account, this lens is for you.
4389096
This is my 2nd lens in the Olympus Pro line; the first one is 12-100 f/4.0. I used the 12-100 quite extensively for travel late last year and it will continue to serve as my primary lens for travel. However, for other times that I don't need the range, I want something faster. While I do have a good collection of fast fixed FL lenses, I am still primarily a zoom user, so I bought this 12-40.

The immediate benefit is a faster lens and as well as smaller and lighter than the 12-100. It is well known to be a very sharp lens and indeed it is. Wide open is very good, no need to stop down much to gain extra resolution. Performance is very even across the entire frame, not just at the center. Resolution is a little weaker at 40mm, but that is only in comparison to the excellent performance at the shorter end. AF is instantaneous on e-m5.2. Have not tried AF-C because I have yet run into situations that I need it. Build quality is very high, consistent with my expectation on Olympus Pro line.

It is, however, not an inexpensive lens. I have not compared it to standard slower kit zooms, my recollection is that many m4/3 kit zooms are not bad at all in terms of sharpness. The main advantages of this zoom are much faster aperture, WR and much better build quality.