I've got the tamrac adventure 75, and while I haven't had a whole lot of other camera backpacks, I can attest to the fact that it's the perfect size to slip underneath airplane seats. it's not weatherproof but it's fairly robust (I've had it out in the rain on a number of occasions) and has a more-than-decent amount of storage, and is also pretty reasonably priced.
I typically throw a 5DII (no grip), 70-200 f/2.8 L IS II, 24-70, and either 16-35 or 100 macro in there, along with CF cards, cleaning kit, my sunglasses, some airplane reading material ... etc. if I've got the 100 macro in there, I can even fit the 50 f/1.4 as well, since the 100 macro's hood is far more manageable than the 16-35's ridiculous lens hood. I'm not even sure how much of a difference having the lens hood on the 16-35 is, but it's just habit for me to keep it all together.
to be honest, I'm not 100% sure that I have the Tamrac Adventure 75; Tamrac issues far too many backpacks that are almost the same size but with different names. I'd say stop by a local shop and rummage around; try things on to see what's comfortable. some of the newer bags have padded straps that are really, really comfy, and that's priceless if you're going to be hauling gear around all day