I've got this rack/bag combo too, and I am happy with it. I carry my work clothes, shoes, towel, small shower kit and some spare clothes (extra socks, gloves and whatnot) without filling the bag/panniers to capacity.

I'll also mention that Topeak sells a rain cover for a little over $10 that slips over the bag (and panniers too if you have them extended). Very handy for winter commuting here in the Pacific Northwet.