I have a
Thinktank SpeedRacer, stupid name, great bag. I've got any number of bags in my bag graveyard upstairs, two rucksacks, a couple of sizes of shoulder bag, one of the waist belts that you attach bags to (and plenty of bags to attach) and I've had a slingshot that I gave away.
The Speedracer is the first one I've really been happy with, it'll happily take a 5D with battery grip and 70-200mm lens fitted, along with a couple more lenses, flash kit and extras like spare batteries. It has a comfortable shoulder strap and a nice, thick padded belt, it will slide quickly round your body. I tend to have it slid round to the back when I'm walking a long way and at the side when I need to get to stuff, I never felt comfortable with the way the slingshot did that (and dropped a lens once trying to get it out).
cheers
Dan