The Incase Compact Backpack (US$79.95) has been a surprisingly pleasant and functional addition to my commuting around the city with my 15-inch MacBook Pro.
Unfortunately, my MacBook Pro doesn't fit in my beloved Jost leather satchel. So, previously, I'd sling the satchel over my shoulder and put my MacBook Pro in a padded sleeve for my odd excursions out of the house with it (clutching it tightly to my chest with both arms, shielding it from the potential harms of public transport in London).
More recently, my work has required that I'm out and about a whole lot more, and with a few near drops and knocks, I quickly resolved to get a decent backpack.
Specs and performance