I have a G-tech pro and it was worth it. I got it for free awhile ago on a deal from Tire Rack

They are high on MPH (or track is low, since track MPH is avg over last like 60 ft), and are relatively accurate. They may measure a little fast/slow, but are very consistent and can be used to judge changes to your car pretty accurately. Important things are to use as level a road as possible, go both directions on the same stretch, make multiple runs, and average them. It might even be worth buying, or the new competition one, if you get a good deal or you can't get to a track easily. I kind of like mine, since I've never been to the track