I can't say much about crumpler, but the chrome messenger bags are great. I use the same one you are looking at (the metropolis), and i absolutely love it. it is a little on the expensive side (like pac, etc.), but it is waterproof (rest assured on that), and durable as hell. I figure mine will last a good 6-7 years (hopefully) and i use it everyday. So i do recommend the bag ... but take that for what it's worth, since i haven't tried the others.

as for the back issues, i'm one who will say their aren't any, but everyone is different. The first few weeks were a little bit rough, but my back strengthened where it needed to strengthen, and since then i haven't had any problems, unlike with my backpack which caused all sorts of problems. Just be sure to keep it tight and where it's supposed to be on your back instead of hanging down real low (the buckle keeps you from doing this anyway).

good luck with the choice.