I do it, but follow many of the caveats already listed in this thread. I only do it where I know the road, this obviously doesn't account for random debris in the road. I also don't draft directly off the back of the bus/truck/whatever and try to stay to the right side. This allows me to see some things coming at me as well as hopefully puts me directly in the bus/truck drivers right rear view mirror, but I'm not sure if that really matters.