Depends. I have three packs and the Topeak DXP Trunkbag. I dropped the rack and trunk because I did not like how it made the bike feel a bit unstable. Plus I would always be paranoid of the trunk falling off -- it never did but was just a mental thing.

Since i use a smaller 20" wheel folding bike to hybrid-commute. I'll either use a Chrome Citizen Messenger, Maxpedition Pygmy Falcon II or a Burton backpack.

Fully loaded Chrome


Commuting Setup: Chrome Citizen Black - Internal Contents by sudoshift, on Flickr

This is pushing it. Loaded quite heavily.


Pygmy Falcon II Work Commute Contents by sudoshift, on Flickr

The Maxpedition backpack is great because I can put water on one side of the mesh compartment and tools in the other.


Maxpedition Pygmy Falcon II by sudoshift, on Flickr

My favorite has to be the Burton.


Burton Backpack by sudoshift, on Flickr

It looks great and its highly visible since its yellow. Downsides it gets really sweaty.