I cannot sing the praises of Tom Bihn enough.
www.tombihn.com.
Fabulous bags, so very very well thought out. I have the brain bag and it holds my Macbook Pro, my QE2s, big bag of cables and flash drives and power supplies, my pashima, a book, DVD case, a pack of clothes incase regular luggage fails to make it, my medicines, etc. It's completely wonderful top to bottom.
Like
limeyx a couple of posts up, I also use my Tom Bihn bag for a 17" PB. I like it so much as a backpack that I sometimes use it (minus "brain bag" and laptop) for hikes in the White Mountains when I don't need the big Gregory. The straps do a great job of getting weight down on the hips. You can get cheaper bags, but I don't know of a better price/performance mix.