I'm using the Lowpro Orion TrekkerII, and I find that it meets my need reasonably well. I can fit a Canon 30D, Canon AE-1 and Linhoff Technica backs, with a few short lenses if i'm being really adventurous. Also the additional meshing and zippable pockets in the top compartment are added bonuses since i can keep memory cards, manuals, filters and misc in there.
There are a few things that this bag lets me down on though. The top compartment is big enough to put a rain shell, or a small lunch, but not much else. It is infuriatingly just too small for a laptop. Also, the camera compartment is too small to put any large lenses in, and forces me to carry the 70-200 in the top part, as well as the 580EXII.
Lastly, the bag is pretty comfortable, and i have been able to hike with it on me for a couple of hours, fully loaded, with a tripod (super light one) hanging off the back to boot! However, to get the bag comfortable you need to have the chest strap clicked in, or else the straps uncomfortably hang too wide in my shoulders.
All in all, for what it does, I love this bag as an all-rounder. Wouldn't be good for real serious types, but it does the trick for me.