I recently abandoned my garment bag that hooks onto my rack because even with the rain cover on it my stuff got wet. I think it's partially because the rain cover had holes in it for some reason that I could never figure out (manufactured that way - holes on the sides yet no drain holes on the bottom - wtf?) and partially because the bag was so low to the ground that whenever I hit a puddle, it would get soaked.
Now I use a Chrome Metropolis messenger bag and NOTHING gets wet - it's rained almost every day since I started using it 2 weeks ago and I ride every day. That bag is amazing!! I'm thoroughly impressed and would highly recommend it.