I've tried all sorts of options for carrying commuting gear. Personally, I don't what to carry any more gear than I have to, so I leave heavy and bulky items in my office (eg, shoes, pants, towel, shampoo, etc.). What has worked best for me depends on the bike. For bikes with rear racks, there are many racktop bags that are easy to mount, remove and carry. I recently bought a Louis Garneau R-16 bag, and it is just the right size for my needs and only cost $35. It easily attaches with Velcro straps, and has a removable shoulder strap for carrying if you need it. I like this bag so much, I added a rear rack to another bike so I've got two bikes that I can use it with.

On my other bikes without rear racks, I use a Carradice Barley or Pendle seatbag. Those bikes actually have small "Bagman" racks that attach to the saddles and help stabilize the bag. The Bagman racks also have quick releases that make it very simple to attach and remove the bags.

BTW, if you have a Tubus Cargo rack, then Orlieb panniers are extremely easy to mount and remove. Tubus and Ortlieb are owned by the same parent company, and they have designed the racks and panniers to be compatible. However, I have found panniers to be overkill for commuting 99% of the time. Why lug around panniers when you really only need to carry enough items to get you through a workday?