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.
I have a Tom Bihn too that fits my 17" powerbook (forget the model of bag) really good quality but not bargain basement pricing if I remember.
laptop carrier also is removable for me if I am not carrying laptop.