I personally can't stand riding with a backpack -- I just find it uncomfortable to have weight on my back while riding.
These days I commute with a trunk bag on my rack. It has a bunch of pockets, so I keep it permanently stocked with headlight, sunglasses, repair kit, and a lightweight cable lock, so I'm prepared for anything ;-) Plus it has room for my lunch, spare clothes, etc, though I don't think it would fit a laptop.
I also have a set of trek "grocery bag" panniers, which I use for larger loads. They're ok (and cheap, $70 for the pair), but they have a tendency to fall off the bike. If I was buying panniers now, I'd get something a little pricier that attaches securely.