Only used twice or thrice (a long time ago) because I like to carry a larger bladder.  It's really nice - keeps your back cool along with the liquid.  I piss-poor pic can be found here:

http://gainesville.craigslist.org/bik/3296770660.html  
(I am not really selling it for $5, I simply could not shrink it enough to fit on Jayson's site)

I have cleaned first with baking soda and then with bleach; so don't worry, you won't catch anything to make you as cool as me.

mk