Non contact digital small engine tachometer

Does any one have experience or suggestions regarding non contact digital small engine tachometers , how accurate and reliable are they?

I see them on Ebay as low as $15 and as much $170.
are you looking at the optical ones? I used to use them on my model airplane engines and they work well. Post a link to the one you are looking at.
The type of tach i was looking at resembles a non contact thermometer which reads via a beam which "bounces" back to the source and provides a reading.

Thanks for your replies