I've been using satchels (i.e., a soft flat bag with a shoulder strap) and similar things to transport documents, books and a laptop for ages, but I decided to switch to a backpack because the bags I own are awful to carry on a bike (which is my main mode of transportation if I'm not too lazy and take the bus) and are either too small or too large for the stuff I usually want to carry around.
As I said, it's intended for urban use and should safely hold a laptop. It shouldn't hinder biking (as my satchels do) but still hold it's share of content. It wouldn't hurt if I could use it for plane-travel, but it doesn't have to be particular suitable for extended trekking.
There's a large amount of brands, features and prices available and I would like to know how I can make a buying decision. Are there things to watch out for?
Edit: After getting too much focus on the bicycle aspect, I'd like to clarify; I'm not primarily concerned with biking. I just want a functionally optimal back pack. It is a mere side constraint that it should be bike-transportable.