Cheap tachometer

Has anyone had any luck using those cheap techometers they sell on e-bay for around $10. The type you wrap a few turns of wire around a spark plug lead.
I bought one to do some testing on my KE100, but the readings are all over the place.

Probably trying to read at real time, no delay built-in, if that makes sense. I have a "Tiny-Tach i've had for years but think they are about 70USD. They have circuitry that buffers that induction signal you're getting at the high tension wire. Stabilizing the display LCD.  

That's why i stick with my old analog meter style, can't beat it for simplicity.