My wife and I have Topeak racks on our commuter bikes. I use an MTX bag for commuting to carry small repair items, change of clothes (business casual), and lunch (leave shoes at work). Only in the wild variant temps of spring and fall is space even a thought--pants/long sleeves in the colder mornings but shorts/short sleeves in the evening. Carrying office clothes and the extra set of AM or PM clothes pushes bag capacity to the max, but I've always been able to make it work (2+ years or so). After several thousand miles, my Topeak bag is still in 9.8/10.0 condition--a few small scratches on the reflective tape sewn around the bottom edge.
Also, we use a Topeak rear basket for light grocery runs, trips to the strawberry/produce farm, library, park/picnics, and small-load shopping trips. The basket is also slide-on/slide-off. The Topeak system allows us to interchange either the bag or the basket easily. For some who have used other products, the Topeak route might seem trivial or a waste of cash. This system made sense when we started using our bikes more and more, and the cash outlay is trivial if you're using your bike 5+ days/week like we do. Heck, very few bike accessories are noticeable cash dumps if your bike replaces one or both of your cars (one car in our case).
Finally, I agree with others about lights. If you're commuting in the US after the time change (Nov-Mar), then you will likely be biking in the dark either in the AM, PM, or both. Not an issue during the summer but definitely an issue to consider later on.