Not a bad idea at all actually. Instead of having 12 different gauges and running a bunch of wires through the firewall, you just plug one gauge into the OBD2 port. This might find its way into my car fairly soon
I mounted it below the AC control, and it works great. It does MPG, MAP data, Air Temp, Coolent Temp, TPS data, O2 open/closed loop, and much more for about $170.