I don't know if it's against the rules to post to another forum, but I've had this link bookmarked since I ordered my MBP a year ago:
http://forums.macnn.com/69/powerbook-and-macbook-pro/178702/need-backpack-bag-sleeve-your-pb/
I personally have gone through three different bags:
Brenthaven Metro: A perfect bag for traveling to the coffee shop. Protects the MBP and has just enough room for a few (small) folders, your power supply, and a small bluetooth mouse (such as my IOGear Mini Mouse).
I dropped it because I had pre-ordered...
Targus 15" radius Messenger: Even better in many ways and a bit more stylish. It doesn't have the soft protection that the Brenthaven has, but it comes with a removable sleeve when you just want to carry your laptop. It also has a bit more storage, but not too much.
It also has a nifty pouch in the back for an iPod.
With these two bags, I was purposefully trying to avoid getting a bag large enough to carry my entire library of books and catalog of DVDs, as I had done in the past. Why get a 6 pound notebook and carry an additional 30 pounds of stuff with you? These two bags prevented me from doing that.
However, I then started commuting by bus and needed room for a book to read, a water bottle, and an umbrella (I live in Seattle), so I then upgraded to:
SwissGear Maxxum: This is the perfect commuter bag, which unfortunately does allow me to carry the kitchen sink with me (which means that I often do). However, the side pockets for a water bottle and umbrella are really convenient. plus it's got a small top-side pocket for my iPod and ear canal phones.
I'm sure there are plenty of threads laying about with more opinions than you care to read.
Happy hunting!