I had a successful backorder once a domain that wasn't necessarily very sought after.
With that said, you never want to backorder there. Now that they have TDNAM, a backorder is only successful if noone pays the minimum price on TDNAM. It basically just puts a $10.00 bid on it.
I recommend placing a backorder on a domain you want to bid on, then bid on it until you get it. It will then use up your $20.00 credit.