Jonathon C.wrote a review of Deuter Race X Air Hydration Pack on September 20, 2010
The Great: Lightweight; great water bladder; rain cover; fantastic back-sweat venting; great wallet, keys, phone, camera, snack, rain jacket holder.
Cons: that's pretty much all you're going to fit, maybe a pair of flip flops if you biking and want to walk out of the woods and into civilization for a bit. A bit small, I'm 5'9" 175 and it might feel a skosh tiny on someone much bigger.
All in all, a great bag, have used it a ton this summer, works great on a mtn bike and has a lot more utility than the camelbak-only bags, with not too much more weight. Enough room to carry the essentials, a great water bladder and comfortable design that stays dry in the rain.