Summary: I bought the slingshot 300 because I needed a pack that wasn't too big....but big enough to carry my D300 with at least one long zoom plus 2 or 3 other lenses plus a flash, extra batteries, etc. I also wanted something that would be comfortable to wear for 3 to 4 hour treks (or more), whether hiking, snowshoeing, or whatever, while providing easy access to my gear and at least some degree of weather resistance.
Pros: Comfortable, Easily Accessible Equipment, Great Design, Strong Construction, Well Padded
Summary: As I grow older, carrying my gear in an over-the-shoulder bag is less and less viable when traversing rough terrain. My Lowepro backpack, which I use for longer hikes, is great -- but I don't like having to put it down to get anything out of it. Now the SlingShot 300 is just what I needed: even distribution of the weight, with quick access to the gear. The hip belt is especially helpful in relieving the weight on the back.
Pros: Comfortable, Easily Accessible Equipment, Lightweight, Strong Construction, Well Padded