My kids have the 18 ounce ones, and dh and I each have a 27 ounce one. I want a 40 ounce one for the summer, but they weren't selling that size when I bought mine. When DS4 first got his, the 18 ounce size was a little large. But as hot as it gets here in the summer, the kids really go through the water when we're out and about, so I'm glad he has the larger size.
First of all, we love them! We have had a couple of minor issues with them. One is that for some reason the kids' bottles sometimes bulge out a bit at the bottom. I don't know if it is something to do with putting cold ice water in it or what, but occasionally they get slightly rounded. Nothing that a couple of taps with a blunt hammer won't fix.

The other thing is my kids like ice water, but since these aren't insulated at all the bottle can be quite cold to hold onto. They have the sport lids and were able to carry them by the loop that forms with the thing that tethers the cap cover, but DS6 said that cut into his hand (I tried it and I tend to agree). I ended up knitting and felting covers for them. The kids love them and I've also learned that the covers help keep the drinks cool longer, so I'll be making some for DH and I as well. You can see a picture of what I did here:
http://pics.livejournal.com/poledradog/pic/0000hzrg