I'm a bike commuter and use this bag everyday. The medium size carries my laptop and pretty much whatever I need for school. It is waterproof, durable, useful, and quite sexy. I love it.
That said, I have a few issues with it:
-why no carrying handle? most messenger bags seem to have them; it doesn't take much to add a lot of utility
-the way the shoulder strap attaches to the bag should be improved. my strap has inadvertently detached from my bag on several occasions. It really sucks when you're quickly walking across a busy street and the shoulder strap comes out of the slide. Swiveling snap hooks, clipped to D-rings attached to the pack make a lot more sense to me. Additionally, they would prevent the strap from getting twisted which happens to me often enough to be a little frustrating
In short: great pack, just rework the strap and add a carrying handle