I bought the flasks to decorate for a bachelorette party. They worked well for everyone and it was a great deal! There were a couple that were difficult to open initially, so we had to use pliers to open it. As far as decorating them, the smooth, clean surface was great to work with.
Nowadays I've been keeping one for personal use, and buying additional flasks for the purpose of collecting signatures at events I've been going to. I do somewhat wish that the surface kept permanent ink a little better, but this is such a minor nitpick that it's really not worth considering for most users. I highly recommend this product!
I've never been able to find a fault with this flask. I've purchased two of these, and having owned the first one for just about two years I can say with great confidence that this is pretty much one of the best flasks you can get in terms of utility and value. It's not fancy by any means, but it holds what you want it to hold, and I've never seen it leak even after storing it in a freezer a couple times. In my experience it does exactly what it's supposed to do, and does so flawlessly.