Not ideal for sit-ups, great for lower back support in the sofa
598438
We have a bit of a unique use of this item.

My wife bought this item a year ago as a support to do sit-ups. Unfortunately, she did not like it at all; too hard, and the curved design makes it uncomfortable.

After months of taking the dust in a drawer, I ended up using it a support for my lower back in our sofa.
I have some lower back pain and this does a fantastic job.
542163
Helps with lower back issues, and posture for persons who are seated for long periods of the day.  This design is thicker than the cheaper competing product my coworkers use.  Has a zippered exterior that can be removed and washed, and the cushion returns to it's pictured size when you get up to walk around.  That should still be in your plan once per hour.

Update:  I have been using this cushion for several weeks now, and the biggest problem is the non-slip surface.  It isn't non-slip.  I use a cloth covered office chair and this cushion slides all over the place.  You have to kind of get a running start before sitting down to keep in where you place it, or it will be off to one side immediately.  Another issue is that it flattens out too quickly.  I am 5'11", and 180 lbs.  After one hour of sitting the cushion is completely flat and you are feeling the surface of the hard chair beneath it.  This may not be an issue if you are smaller than I am.  But I am reviewing the extreme comfort cushion as well, which I recommend by far over this one.  It's also $!0 less expensive.  You will find with these that it takes a few weeks to evaluate them correctly.
3410060
The picture is different than the sofa we received, somewhat disappointed because you cannot sit on it with out sliding forward it does not lay flat in the sofa position. The bottom cushion is angled and so  is the back cushion at about 30 degrees forward. I would not recommend this if you want it to sit on.
898015
Barely used but when I attempted to use it for my lower back it was too stiff so not really good for Lower back exercises.
816613
We replaced the spring mattress with this 5" mattress and used cardboard underneath for support as another person suggested but my 5'11" 170 lb son still felt the "bar" in his back.  Requested a return but supplier deleted my request for untrue reason of "duplicate return request".  Amazon A to Z Guarantee folks called in.  After 3 more days (total 9 days by then), we cancelled the return request and donated it to recover the space.  Replaced the sofa bed entirely with a queen platform and 14" foam mattress from another source.  Would have given higher rating of 3 stars because 4" of base and 1" gel foam was a gamble to begin with, but only 2 stars are being given because of bad experience trying to return.  If you are haunted by the bar in your fold out sofa bed, if your couch is like mine, it may not work.
3145632
I have purchased three different cushions in the past few months to help with lower back issues.  The quality of this item is wonderful and it is both firm and comfortable! However it is uncomfortably narrow I'm hanging over the edge on both sides unless I reposition myself every couple of minutes while driving. Although that does not cause pain, it is annoying and I haven't had this happen with other Cushions  I've used.  I did not read the dimensions carefully enough. I am of average build, right between women's pant size 6 and 8 and not pear-shaped.  I'm hoping this manufacturer makes the exact same density and something about 2 inches wider so I can look to replace.
4535454
This is a good quality mattress pad, and definitely a good option for those who don't like memory foam. It fit the bed well and added a lot of cushion to my new, but slightly too firm mattress. It didn't work for my needs, however and I actually gave it to a friend. After one night using this, I tossed and turned and woke up with terrible lower back pain that had otherwise gone away since replacing my mattress. For me, it added plenty of cushion but not enough support. Luckily my friend loves it. This is a good alternative to 1-2 in memory foam toppers and it does stay cool, but it won't provide as much support.
706078
I use it to support my back on air and car travel.  I find that regular seat backs just sag too much making my back feel like that of a turtle.  The Therarest pad gives the seat backs just enough stiffness.  If it was 2-3 inches longer and a slight curve for the lumbar, it would be perfect!
688571
I have the "wedge" version of this, which has a slightly different utility than the "cushion" form. It's silly to me that this isn't wider.

The cushion form is actually very versatile because you can use it for balance exercises on the floor (if you look for "balance cushion" on Amazon, you'll see examples of this in use). Possibly, I'd also think the cushion form is not wide enough for my druthers, but I see the argument that it's like sitting on a ball, and so maybe the localized rounded surface is like that.

For the wedge, however, it's decidedly planar; you are sitting on an inclined surface. And, darn it, that surface should be as wide as your chair. The seat of my desk chair is 19" wide, which is also true of my dining room chairs (I end up working at the dining room table a fair bit). Since I am not petite, this means that parts of me are hanging off the sides of this thing. And there's basically a corner there at the edges. So, unless you are tiny, that's not something you're going to be comfortable sitting on for any length of time. (The effect is mitigated if you put a pillow on top of this, though.)

Pros:
1. The idea is clearly great as a "help sitting up straight" thing, and I do like that facet of it.

Cons:
1. At 13" wide, this is not wide enough for most adult rear ends or for most chairs.
2. It comes with the needle for inflating, but should probably also come with a pump. These things do lose air over time, so re-inflating is part of the deal.

Other:
1. The textured surface is shown pointing up in the pictures, but I think it's more useful to point it down, so that it's less likely to slide off the chair. For the balance cushions, this is how they're often shown in use. (Although the ones I used when I was in physical therapy didn't have the little buttons, and didn't seem to slide around.) I would also not like to sit on that texture for long.

FYI in case you're considering the cushion version that the physical therapy exercise I did was to stand on the cushion and then move one leg out and back in to the imaginary numbers on a clock. There's lots of things you can do with them; just google "balance cushion exercises". Some of these could be done with the wedge, and some call for a larger cushion, but they're actually useful things for exercise.

Actually, when I think about it, if you want a wedge, you probably don't really want/need the inflatable facet of this. It doesn't really give you the same experience as a balance chair at all. Even being narrower than I think it should be, it doesn't give you the same "constantly working your core to stay balanced" experience of a ball.

Update: The comment from Gaiam folks reminds me to mention that I have other Gaiam products, and I think they are a fantastic company. I just don't think this particular product works.
3100784
It's a good product. Unfortunately it didn't work for me. I have a low back spinal fusion and I bought it to put under my knees when I sleep. It's unfortunately not thick enough for my needs. So I am going to use it to make me more comfortable in my car.
254273
This mattress has really good reviews which is why I got it however its really only idea if you're a stomach or back sleeper only! I am a side sleeper and the mattress does not allow my body to sink enough for my spinal cord to be level therefore I wake up with tons of back and shoulder pain. Im trying to train myself to sleep on my back just because its a pain to return.  I assumed perfect cloud meant it would have a lot of plushness and sinkage to it but its pretty firm.
2898637
I love this product! I bought it to help improve my bad posture, and it definitely does that! It isn't so tight that I can't breath, it's just tight to do some subtle shaping. To me, it feels like a tight hug. I like that there are three rows of eyelets so I can decide how tight I want it.

However, it did take me a while to "break in". Also if you don't have a long torso this product might not be for you. I do so it works for me. I didn't like how the lines do show under some clothes. If you want it for waist training purposes I don't know how well this product would work for that.

I just really wanted it for posture reasons, plus it got rid of a stomach pooch I didn't realize I had! Overall I love it!
31 inch waist-size large
4475106
I use this mainly for my car seat and it works perfectly. It took some playing around with the seat position, but I eventually got it just right. I place the cushion close to my lower back, but not quite touching the base of the seat. I find that this offers the most lower back support without compromising my neck posture.
4860715
I bought these to wear under white jeans/capris because I didn't want a line across my thighs from bicycle short type items.  The length is great, and unless you are wearing something form fitting to your ankle, you won't see the line/bump at the end of the legging.

However, I didn't realize that these are slightly high-waisted.  I have a true high waisted shaper that goes up to the bottom of my bra, and other than a slight foldover (very slight) it stays in place.  Well, this one hits maybe the bottom of your rib cage and is fine until you stand up and walk.  Then it rolls over several times and aside from creating a big muffin top, it's also pretty uncomfortable.  Luckily, I have on jeans and a loose linen tunic, so nothing shows.

If you get these, I would advise not trying to really pull them up as far as they go on your ribcage.  If you keep the top edge a bit lower, it's less of a rollover and more comfy.

Also, I wear a size 10 pant most of the time, and these are super snug!  Worse than the L'Eggs super support panty hose.  You might be able to go up a size.
4906579
Like most reviewers, I agree with the pillow seeming super hard the first few nights of use.  It definitely took some getting use to, but I really liked the support and thickness it provided.  However, this pillow is not great for people who sleep on their stomachs.  I usually fall asleep on my side and then, at some point during the night, roll on my stomach.  This means I wake up with a horrible kink in my neck and knots in my shoulders because my head is too high off the bed in relation to my neck and back.  I pretty much have a love/hate relationship with this pillow.  It's super awesome for when I sleep on my back or side (and stay that way), but if my sleep self has other plans... it plain sucks.
4155056
I have two pairs of these and all in all they are great - not for every day use but more for days when you are feeling (and seeing) your muffin top a little more than usual. It's great for wearing under jeans as it smoothes and flattens the tummy. My main issues with this item though are (1) it rolls down after a while and you have to adjust it which can be a bit annoying (and embarrassing in public) and (2) you can't wear it for extended periods of time as these run quite tight and can feel like they're cutting your circulation after a while. It is definitely an item to have in your closet especially for wearing under a fitted dress if you have a larger mid section.
1103672
I got this to use my laptop on the couch when watching TV at night to get a few more emails attended to.  It turned out to be a little too high and awkward.  My couch is leather so perhaps the stands are expected to sink into the cushions more?  However, I see potential uses for this so I'm not sending it back.