I've had a M.U.L.E. for four years now and it's the best. Had a friend that wore his on day cycling trips and was always jealous, so got one of my own. On a day hike it can carry enough water and snacks to keep you going. You can adjust and configure to carry just what you need, as long as you don't need more than the three liters of water. (I'd stick with the three hour recommendation, but as long as it's not too hot and dry you can go a little longer...especially if you start your trip well-hydrated.)
The air director back panels really do work well and help with keeping your back from overheating.
Great, great small pack. Highly recommended.