I have this pack and love it. I bought it when I was in college. It is specifically designed to hold a 15" MBP, has a built in rain cover, and is really comfy. This model is discontinued, but their site has some other versions. They are really cheap on ebay. In the store mine was $120, but I only paid $40 on ebay. I have used and abused mine for about 3 years now (on the bike and off)

http://www.stmbags.com/catalog/lapto...ptop-backpack/