I'm currently using a single rear pannier from Mountain Equipment Co-op (you can check it out
here) for my commute. It works pretty well so far (I'm pretty new to this myself). My only complaints are that my clothes get wrinkled and it doesn't seem to be ideal for a laptop.
I'm considering getting
this commuter bag from Two Wheel Gear. It seems like it might solve my wrinkle and laptop problems. I haven't tried it yet, though, so I can't offer feedback. Just thought I'd put it out there as another possible option for you.
Good luck!