I made the decision that I wasn't going to worry about getting a totally waterproof bag for commuting. Instead I bought a drysack (also Osprey) for the laptop and a cheaper Karrimor one for my clothes. However, I've commuted through some fairly terrible weather and the Osprey backpack hasn't yet let any water in. I'm glad to have that extra security, though, and a drysack is a much cheaper option than getting something like an Ortlieb backpack which is guaranteed 'throw it in the river and it still won't leak' waterproof.