I use a Jandd, which I love. It's pretty compact, but what I love most is that it has a cover that rolls down over the components, so when you're off the bike you can carry it like a regular messenger bag. I got tired of going to dinner or movies after work and having the little hook on the bottom attach to random pieces of furniture as I went by. I think its the Commuter Pannier (though mine's old, so might not be exactly the same) on this page:

http://www.jandd.com/search_results....el=2&subcat=11