Yeah, I switch leads, and also switch which shoulder I'm throwing over. I'm not sure about the bounceback. It's really all about getting it up, over, and accelerating downward as hard as you can. A solid surface (i.e. concrete) will absorb and disperse the force by cracking/denting. Then you just pick up the sledge and get it going again.
I'm actually wondering if the bounceback might make the workout even tougher due to the fact that the energy isn't absorbed, it is actually redirected. As it bounces back, it seems like you'll be forced to expend energy controlling it so that you don't lose it off of the bounce. Then you'll have to reset and get it going again. So, rather than getting a little rest immediately after impact, as with concrete, you'll actually have to exert yourself controlling the bounce back. I'm really not too sure if this'll hold true or not, but I think that I might try it out.