You should check out the Ortlieb Office Bag. I have one and love it. I use it on an 8 mile daily commute to carry an IBM Think Pad in a laptop sleeve, a change of clothes in a packing cube, my Canari rain jacket, and a bunch of misc stuff like wallet, cell phone, keys, extra sunglasses, and sometimes a bike lock or lunch.

It looks nice, it's waterproof, and adapts to a variety of rack configurations. I take it with me into my office, and I find it easy to tote around. However, I don't use the shoulder strap, so I can't comment on how it works as a quasi messenger bag.

Also, it sits at an angle when attached to the rack so it's out of the way from your heal while you pedal. In fact, I don't even know it's there when riding, and that to me is the ultimate test of a commuter pannier. I'll try to post some pictures of it later.....