I have the Arkel XM-45 and XM-28. Both are very nice. I don't know how they compare to other bags, but what I like is the full-length zipper that allows you access the gear you need, which seems to always be in the bottom of the bag. No unloading of the bag. Pricy, yes, but I liked their design.
I agree with cyccommute about using ziplocks for internal organization. Not only do you get the waterproofing, but it is awfully nice to just grab a bag and head to the shower instead of rummaging through your bag looking for the shirt, the socks, the shorts . . . .