When you wear on cap it leaks water a little ...
340687
When you wear on cap it leaks water a little, but when you wear both of them it not leaking.
403362
I loved it at first, but
Then I found out that
If you misalign the cap, even a little, by
Such a small
Amount that the misalignment cannot be
Perceived,
It will leak. I kept using it
Even after it leaked for the first time, but it
Continued to leak unexpectedly.
Even if it is closed tightly and feels secure, it could still have a small
Opening
From the cap being misaligned.
So what I'm trying to say is, if you
Have electronics or other things that should not be getting wet
In your bag with your water bottle,
This product isn't for you.
2978982
I bought this for my son for baseball.  He loves it, keeps the water cold the entire game and is large enough that he doesnt need to refill during the game.  However, the plug somehow came out of the top clear flip part of the cap and the bottle leaks whenever it is tipped over.  He has soaked his glove and the floor of my car several times!
1179319
Although it does a better job of staying on my head compared to other swim caps (specifically the speedo ones for long hair), I still get water in my cap, especially if I'm swimming on my back, which is very annoying. I do get a lot of compliments on it's unique look, though, and it luckily does not come right off my head and into the water like the Speedo ones.
4229006
This cap is great. You do have to make sure you dry it thoroughly and maybe put baby powder on it every now and then to keep it non sticky.
It doesn't prevent your hair from getting wet though, which is not terribly bad, but... if that's what you are expecting you won't be happy
4166398
Weakest part of this water bottle is the cap. i bought two of them to keep me going for now.
1845186
Love everything about the bottle except that it leaks out the bottom of the screw on cap. Every time I take a drink I get drops of water on my lap and the top is screwed on correctly. I had to stuff a Kleenex in the sleeve to catch the leaking water. If I was using this at the gym it probably wouldnt bother me as much but I am using this for work and that just will not work. Will be returning...
4611810
I love the size of this water bottle and the fact that it comes with three different caps. The only problem I've been having is it leaks if you don't have the cap on extremely tight - so tight that you can barely get it off. I'm not sure if I have a bad seal or if they are all like this but that's the only negative.
4452014
I was aware that this cap was not back pack safe, but it leaks a lot more that I anticipated, and not just from the sports cap. It leaks from an air hole that seems to have been "sealed" with hot glue.
4422369
It's not bad for what I need it for (watering dogs on the go), but the spigot leaks a bit when it's just sitting there. Not sure if I can return it now that I've actually used it and found out. Very glad I only had water in there.
2667200
It goes on easily, however the part that goes over your ears doesn't fit well so it lifts and water gets inside.  Only the top part of my hair remained dry.  The search continues for a better swim cap.  I may send this one back.
2947426
This product is alright but the cap on the bottle I received leaks, gets air bubbles in the water stream, and makes a weird hissing noise when I drink from it.  It's still functional but it's irritating so I barely ever use it (which is a bummer because I wanted to use it every day to monitor my water intake).
1199598
Really great except that it has a little nub for letting in air when you suck on the cap which sometimes leaks a little.  It's the perfect size for a short hike, the loop is really convenient, and the metal itself is durable.  The slightly leaky cap knocks two stars off, though, since that's one of the primary purposes of a cap.  The leakiness is not that bad, but I wouldn't put it full into a bag of things that are not water-resistant (napkins, maps).
3875657
When this cap is closed, it seals the bottle fine. But when you turn it to drink, the whole thing comes apart. Did anyone test the design of this?
4756405
The bottle that I received has a defect, when the lid is closed if it tips at all, the rubber seal inside the cap does not actually stop water from flowing out of the bottle. In fact the cap literally fills with water so you'll get a big splash when you turn it up.

Edit: my bottle was replaced by the company and the replacement met my expectations.
2372078
Hair still gets wet, but it is a good quality cap.
3959640
Love the bottle hate the leaky cap.  Suppose to be leak proof, sometimes it is and other times not, more times not.