My personal favorite is the Topeak MTX Quick-Track pannier/trunk bag combo along with the matching rack. It's a very roomy trunk bag that folds out into an even roomier set of panniers. Best of all, although you'd have to swap out your perfectly good existing rack, this bag/rack combo allows for very easy removal and remounting of the bag so you can simply take it with you when you run errands. The MTX bags come in different sizes and with/without fold-away panniers.

I must have spent over a year shopping for the "perfect commuting panniers" and, in the process, I drove a lot of people on this forum crazy. Now that I've made the decision and had them for some time, I don't know why I ever questioned buying the Topeak MTX and rack. It's a no-brainer, as far as I'm concerned.