Ive got the original version of this (10yrs old now, yikes!) and its pretty water proof, used it pretty infrequently tho so its not exactly worn out. Used it today to transport laptop to/from work/uni/home in a bad downpour and nothing got wet. The flap fabric is very reflective and waterproof, the main bag material seems to be quite waterproof, its quite thick, tho im sure it would probably soak in eventually in a prolonged downpour. Main issue obviously is the fact the lid is just a flap, so you can get rain in if you stuff lots in the bag, a laptop, jacket and some files is easilly swallowed tho so not had any water issues as yet.
The problem i have is the zip on the flap of mine is quite poor and breaks often, also, put a laptop in it and its quite uncomfortable, not padded enough (but then i cant get comfy using a courier bag anyway, how/why do people use these things? honest question! any tips?)