This was a good purchase, and I give it a 4.5 star rating.
I had heard of these kinds "water bags" quite a number of years ago, and wondered, it being 2010, if there were any affordable ones that would fit the needs of the occasional traveler. What I wanted is something that would carry no less than 20 ounces of water, thin enough to carry easily in luggage, or under clothing, and under $30. I have traveled with small 8-20 ounce water bottles in the past, but it required carrying a bag that ended up fatiguing my shoulder.
This works on all three accounts. Yes, I got it for under $30, AND from amazon. It carries just about 2 liters (~67 ounces=less refills = 1/2 star), and can be flattened easily. In fact, I wore it under my coat several times on the plane instead of packing in my luggage. No worries of spilled water, no trying to hold it while you doze, no 10 minutes of under-the-plane-seat shifting while you find room for it in your carry-ons, no more unscrewing tops to take a swig, no looking around while pulling it from your bag while climbing walls or hills. Now that's a 4 star product. Plus, because of its shape, I can get it almost completely full using the airport water fountains, regardless of how high that little spigot squirts. (try THAT with a typical water bottle!)
My only complaints are really wish list items more than real problems, and are minor issues.
1 When you fill the bag, there is no Real way to fill it completely. The opening's bottom edge is almost as full as you can get it, unless you have more than one person holding the bag and getting the water. But you can get pretty close to that 67 ounces.
2 The tube where you suck your water doesn't tuck into anything; it swings freely. That can be good (easy access) but that also means the same thing you stick into your mouth will come into contact with airplane seats, dust, passer-bys, dirt, coats, etc.
3 You can't drink from it in a completely prone position unless either you or the bag are at a lower position.
IF you get this model, please pay attention to this point:
The tube from where you suck water locks. You must pull the blue tube down a inch or so to unlock it. This is a good thing, (less leaks) but only if you know about it ahead of time.
Others commented on this, but only when I started using it did I understand what in the world they were talking about. Practically speaking, it means if you don't unlock the tube, you can suck (HARD) all day and get 1/2 cup of water from your very full bag (yes, this actually happened).