Dirt bike tachs?

[pete-blen]

just wondering if anyone has used one
of these small tachs / hour meter....
self contained..bolt it on/ rap the sencer wire around
the plug lead.. earth the ground wire and yer good to go..

So has anyone used/seen them..

http://www.tinytach.com/tinytach/index.php

[Grumph]

Used them on a couple of classic roadracers. Quite good - reliable. The RPM tracking is not particularly quick but used to set up gearing it's good enough.
Don't know about use on the dirt, may not be fast enough reacting to RPM changes.