Great backpack. I bike commute in Seattle and so far this backpack has kept everything inside bone dry (the reflector is also really bright). My last backpack, which was also waterproof, had so much fabric on the outside that it would dry slowly once it got wet, leading to some nasty smells - the BackSak doesn't have that problem, so I'd recommend it for other bike commuters.
It's got good padding on the back and broad straps that are comfortable for long periods of time. It's also huge - tons of space for everything inside. The chest buckle is a little too high and not really useful for much, but other than that I'd say perfect backpack.