I used to commute several years with just one pannier. I wasn't bothered by the balance issue, I could only feel it if I topped the pannier up with heavy groceries on my way home. Groceries and errands were the reason I moved into using two panniers. Most of the time I could fit everything in just one, but the extra space came in handy often enough.

Currently I have a Carradice saddle bag. I can fit my commuting stuff in there, with room for a couple of items from the grocery store. I now do majority of my grocery shopping during weekends.

--J