I made a thread on this topic a while ago in Commuting and got some helpful replies but looking for more.
Basically, I'm looking for a nice pair of panniers that I can use for some light touring, but also just have one of them I can use for commuting. For the latter purpose I'd like one with a messenger-bag type strap (I don't like backpacks too much) and one that's waterproof although I guess it wouldn't be so bad to just have one that comes with a little raincoat.
Anyway, I've already looked at the Arkel Commuter - nice although a bit on the massive side - and the Ortlieb Back Roller Plus - nice, although doesn't have a strap like I'm looking for.
I've got the Topeak rail rack with the Topeak bag. It zips up on the side if you don't want them to hang down by the wheels, and it slides on and off real easy. I recommend it. It does that have messanger-bag type strap.
Actually, the Ortleib Back Roller Plus has a shoulder strap. Way it works is that the shoulder strap doubles as the strap that keeps the rolled top part rolled up and compact. There's a second strap that goes across the rolled part to keep it closed, even if you are using the shoulder strap.