Since I schlep around our city all day without a car, a backpack is my preferred way to carry camera gear. Sling bags and messenger bags, which go on one shoulder, can be tiring and make you feel lopsided after a while. I also prefer a design with separate compartment for non-camera items. This is particularly handy as a new dad -- you can put diapers and snacks and spare changes of clothes and whatever else in there.
The major downside is access to your camera equipment. I currently have a Crumpler "Sinking Barge", which has a gusseted lower compartment for camera stuff -- this opens for easy access but won't spill everything out. You can kind of swing it around on one arm to do this without taking the bag off, but it's a pain.
The other downside is that a bag which will also include room for a laptop is usually unfortunately large, and can be unwieldy on a train or bus. The Crumpler is super-well made and very comfortable to wear, but it's huge -- and the model I have is the smaller one in its line.
Smaller backpacks exist, though. Before I had a larger laptop, I used a Tamrac Express Pack 8, and a shoved my Sony Vaio U101 into the top. Much more compact, and I still take that out when I don't need my laptop. It's small, but I can fit my Pentax K-7 with an attached prime lens and two other small primes into the camera compartment, and a flash and even a collapsible reflector into the top. The access is definitely weird because of the flap pocket, but on the other hand that would provide some protection against theft if that's a big concern.