I bought mine (Chrome Metropolis) mainly for its ability to keep my stuff dry and to be able to hold a ton of stuff. I'm not a messenger, but I try to use my bike for a lot of my errands and going to school when I can. My largest backpack felt weird with a bunch of stuff in it, and it was a pain to get anything out of it when it was full. I find it to be more comfortable full or empty than a backpack. When the strap and stabilizer are both snugged up, it really feels secure and like an extension of your body.

There are downsides to it too, first being the cost. I've also learned you have to pack it a little different than a backpack due to the way it sits, but you get used to it. Mine is best worn as designed, by this I mean that it's not practical to take it off to carry it with your hand (there is no handle) or just over a shoulder.

Oh, and you will also get a lot of strange looks if you don't live in an area where messenger bags are common or bike commuting for that matter. I get a lot more strange looks and the feeling that I'm being watched and followed by store employees more than when I wear a backpack. If they want to hassle me about it, I'll shop elsewhere.