I usually carry the usual bike tools, phone, change of clothes, lunch, and sometimes a few books (not textbooks) or video games/dvds.  It's good to hear that it doesn't affect handling too much, though I might be screwed either way with the Langster.  I really don't mind riding with a backpack or messenger bag in the winter, but it blows in the summer here.

I may as well just order one.  If it's uncomfortable on the Langster, at least it would still be usuable for long rides/bikepacking on a mountain bike.

Ive been using messenger bags and backpacks for several years, and a few months ago I thought I'd give a rack/pannier setup a try on my commuter.  While it was nice having nothing on my back in a Tucson summer, I absolutely hated the way it made my bike handle.  To be fair, it's probably not the best bike for a rack/pannier (specialized langster).  I didn't have a gps at the time so I can't prove it, but it made me feel much much slower, too.  I'm thinking about getting a big Revelate seatbag for commuting but I'm worried that it might feel really similar to a rack and pannier.  Is there anyone here with experience commuting with one of these who can give me some insight into how it changes the handling?

I should add that I plan on buying a geared cross bike soon for commuting and big rides, and I'm sure that would make a difference.