I have one that I have used for the past four or five years. It works ok. I have never had any problems with it. I don't know how accurate it is, as I don't have anything else to refference it with. There are better tach's out there but they are much more expensive. Mine doesn't get much use, as I fly by myself most of the time. It's rather hard to fly and tach at the same time.
I set my machine up to fly the way I want it and on those rare occasions when someone stops by to watch, I put them to work taching my machine as I fly. They seem to enjoy the hands on involvment and it gives me a headspeed refference for future use.
I wish you well and many happy landings........
Pound for Pound, Heli's are a blast..... Roland