I have the Two Wheel Gear pannier/backpack and have been commuting with it for about a year. It's held up well and works well in either mode. The one gripe I have is that I wish it were a bit bigger (without inducing heel strike). I carry a 15" laptop that just barely fits in the main compartment and can't fit in the laptop sleeve.

I was going to post a link, but I'm too noob to do so.