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I'm using a $80 gps logger that happens to be the same used in the MaxQData system. It didn't come with lap analysis software so I wrote my own. The advantage of a gps unit is that a lot more than just lap times are stored and you can much better analyse you session after the fact. You don't get a real time display, but I'm happy to just look at the data at the end of each session anyways.
Another advantage of a gps logger is that I can create virtual replays of sessions or races from the data. I'm actually in the process now of rendering a movie of Saturday's race 8 with Gunner's data as well as mine. You could be in there too if you had a gps logger as well!!