Good point Kimo2007. The ushiro-geri when done right is a good finisher. The added benefit is that after you are done you are facing away....run!
I know what you mean about the spinning side-kick. It's very common and keeps you facing your opponent when you're finished.
I guess I like the traditional ushiro-geri the best. It probably generates the most force of any technique that I can come up with. It just comes down to finding the right time to let it fly.
keep training, Mark