My comments are basically the same as mrfix's. I have the same Topeak bag (QT Super Trunk Bag DX), and have the matching Topeak QR Beam Rack (seatpost mounted) on one bike, and the Topeak Super Tourist on the other.

The convenience and simplicty of the attachment system is unbeatable. The only reason I can think of for why a commuter wouldn't use this system is because they simply don't know about it. It makes so much more sense to me than a pannier. (ok, I guess you can't fit a full-sized laptop into it...though my Fujitsu P1100 fits perfectly!)

Even if you already have a rack and don't want to get a matching Topeak rack (though I'd highly recommend it), the design of the Topeak bag itself is excellent, and it can be attached to a standard rack. I keep my tools in the top pocket, wallet and keys in one of the side pockets, fill the insulated and padded main pocket with CDs, a shirt, and my lunch. And then it expands if I have to haul extra stuff. Like mrfix, I've used the bag daily for two years, it's still in great shape, and I often use it as a bag for activities and travel completely unrelated to the bike.