I have this rack:

Amazon.com : Topeak Explorer Bike Rack : Sports & Outdoors

And this bag:

Amazon.com : Topeak MTX Trunk Bag EXP Bicycle Trunk Bag with Rigid Molded Panels : Bike Pack Accessories : Sports & Outdoors

I use them on my commuter and they are fantastic. The bag easily slides onto the rack and clips itself in place. The sides unzip and turn into panniers. It's not the cheapest system, but it is excellent for my needs. I use the main compartment for my commute each day (it also unzips and gets bigger if needed) and on the rare occasion I need a lot more room I just unzip the sides and I have tons of room. Definitely worth checking out.