I always buy gravis backpacks. They have a built in comp pocket for 15/7 inchers. I currently have one that is seven years old, and is just beginning to show age. I'm very happy with the product. I will buy a new one whenever this current one bites it. turntablelab.com sells them and theyre on sale right now
For my 13"MBP I use a Timbuk2 Q backpack and it's great. Waterproof, holds more than it looks and it's been superdurable. I would highly recommend their stuff, I know they make bags for the 17" too. http://www.timbuk2.com/tb2/products/home