For the day to day, back and forth to work, I'm still using this removable bag:
Hard bags are more secure and better for trips, but I don't get time for much of that now. This bag holds a huge amount of stuff for work, including all my rain/cold weather gear, and I can take it on and off the bike in about three or four seconds. It's not pretty, but the convenience of carrying everything in one bag is worth it. Cost $20 at the local sporting goods store.
