How tight do you wear the timbuk 2?
If you wear it as tight as it SHOULD be you have two problems. Attachment points dig into your shoulder, and two....its a PITA to get off due to horrible adjustment ability.
it is fantastic for like a 5 minute ride to class on campus or whatever. but do any real biking, with a real load, and you will see it sucks.

just try a chrome bag , and you will see. i can put 30 24oz bottles in it, and not have any discomfort other than it just being heavy. its such an awesome bag. ill get the bigger one when i go back tot he states in feb to bring back with me