I have the Tamrac 759 and it's perfect for my needs. I fit a 20D, D60, 70-200 F4L, 17-40 F4L, Sigma 50-500, 50 1.8 Mk.2, and a tamron 28-200 in it. A lot of the smaller stuff fits in the top two compartments and there's still room in the lower padded laptop slot which I leave empty so it can fit in below the airplane seat in front of me. Carrying this as a backpack can be a little exhausting, but the shoulder strap (additional accessory) comes in handy.
The tamrac 759 was the most suitable for my needs - too bad I ended up spending on the Lowepro trekker with wheels, the photo trekker, and the mini trekker before I found the tamrac.