like the milkman says, the osprey raptor is an awesome pack. I have the raptor 18 and I love it. the bladder has a rigid back that makes it fit the curve of your back so it is very comfortable and makes it a lot easier to fill over my mule. it also has cool features like a helmet holder and a magnet that keeps the hose and bite valve in it's place. tons of pockets for all your stuff. 3 liter capacity. the 18 is the biggest one they make and has plenty of room for anything you need for an all day ride. took it out to moab last fall and it was comfortable with no other issues. recommend it all the way. check active endeavors, i think that is where i got mine.