Anyone have any experience using one of these or anything like it? How accurate is it? How accurately does it read hp/tq?

 

It works pretty good. The hp/tq is not worth a damn. Mine was about 150hp lower than the dyno. Maybe if you had slicks and did a good first gear launch it might work. The times are actually very accurate. Here is a link to the gtech vs the timing lights. Its fun to play with.
http://www.joegorm.com/g-tech_vs_timing_lights.htm