Starlane is the one I've seen more often, but GPS lap timing can be hugely beneficial. I think it's cool no matter what, but sticking the unit on the bike of a faster rider is really where it starts to become eye-opening. knowing how fast you're going in a given turn and when you're braking or getting on the gas can show you quite a bit, but seeing that a faster rider is braking X feet later, and taking a given turn Y mph faster is what really puts it into perspective. I also like the "hypothetical best lap" feature where it will piece together your fastest split through every portion of the track and show you "if you did your best throughout an entire lap, this is what you're capable of running." Awesome technology.