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.
This kept me nice and comfy while boarding in WV last weekend. Granted, it was 50 degrees out, but i just had this and my shell on, and I was very comfortable. A little less soft than I'd like it to be.
This is a big improvement over my initial floppy starter chalk bag. The ring on the the top gives it enough structure to leave it open while climbing and not have to worry about dumping. It is a bit snug, but I have pretty wide hands its no more than a very minor problem.