My husband bought me this as a gift last week and I was so excited and couldn't wait to use it. First, the size - the usable width of the case is approximately 16.1/2cm and the usable length is 21cm - my kindle 4 fits nicely and that should give you an idea as to whether or not your gadget will. The plastic is thick and the seams appear to be secure. There is an adjustable strap.
The fastening is tricky at first - there is a double layer of "ziplock" and at first I had problems getting it to "zip" but that will probably get easier as I get used to using it - once you hear a nice snappy noise, you know you have it aligned correctly. It then folds over and the black top is folded over that and secured in place with the two yellow studs, so in all you have three layers of protection.
Once in use, its easy enough to use all the buttons on the Kindle, they were all responsive, I had no problems with that at all.
To sum up, I could enjoy a good soak reading my Kindle, knowing it was well protected. I personally used it in the hot tub, but of course it can be used in the bath as well and the cover will be handy to protect it against sand on the beach.