I have used Timbuk2, Chrome and R.E. Load in heavy rain. nothing got wet. The Timbuke2 is the oldest and most worn out. It is still waterproof, but the flap seems to be to "soft" anymore. meaning it flaps to one side leaving an opening for elements to get in. The Chrome and R.E. Load seem to be better designed with over sized flaps to prevent this from happening. I commute all summer long, rain or shine, by scooter. I mostly use the R.E. Load (bigger), but that and the Chrome have never let in any rain. here is a photo of my chrome bag sitting in the rain for two hours.

i was sitting at a table drinking beer in the rain at a festival. nothing got wet. not even when the Gods unleashed their wrath on my way home.

But if i were to commute daily with my lap top, i would use a back pack. with the chrome warsaw, or a Seagull bag with a laptop sleeve. I have seen some other chrome bagpacks in use on bikes with lap tops, and they seemed to work great.