I have one of the Raytek MT4's, they are really good for the money. A friend of mine has an Exergen precision gauge that is calibrated (Uses it for work). The MT4 was only a couple of degress off consistantly when we compared the two.

I have also compared the loop style mip gauges and while I have read that they are not accurate, I have to disagree with that somewhat.
Let me explain what I mean, the MIP gauges use Thermistor or electronic element that relays the temperature. The element has a rating in response time and range of temperature.
It is known that in the first generation gauges that there was latency in the time it takes to relay the temperature to the display. This could appear as inaccurate because what you see on the screen is not what you see on the head of the motor. Given a short amount of time however and the display will show the correct temperature.
They had this type of technology in the Personal computer temperature probes back then as well. You could monitor CPU for overheating while overclocking, however in some cases your CPU would overheat before you even have a chance to know about it because of the latency.
I now use my raytek to not only temp my Nitro engines but my personal computer projects as well as other things.
Temp probes have improved in the PC electronics as I assume in the MIP gauge also, but I have not used one of the new ones.

Dbow