I've had a great run for three years (still on its original battery BTW) from my cheapo Tower tach, as seen here...
Most of the time it reads reliably about 6" from the prop. Certain light conditions can cause false readings but these are rare and are obvious when they happen.
Unlike some, this tach switches itself off if it hasn't been used for a minute or two (I always forget to switch mine off).
If you leave it out in the summer sun with the LCD up, it can blacken over sometimes, but it comes right once it is taken out of the sun.
It's been pretty rugged (it's been dropped a few times) and has never given any trouble.
It's well worth the 20 bucks IMHO.