I'm going to medical school in the fall and am going to be walking to campus. I've been carrying messenger bags through undergrad but the length of the walk makes a one-shouldered solution a bad idea for my back/hips. I'm looking for a smallish laptop backpack with a water bottle pocket on the side large enough to hold my 1L Nalgene. It hold my 15.6" notebook, my iPad, and one paper notebook along with the necessary accoutrements (pens, wallet, etc) without getting too bulky. Weather resistance is a huge plus as I am not allowed to park on campus so the walk is mandatory in every type of weather.
Tell me about your backpacks! What you like, what you don't like. I'm searching around on purchasing websites as well and looking at reviews there but I'm worried I'm missing some of the best bags.