I have a harbor freigh tach and it's crap. Sure it looks nice when it's lit up at night but it doesn't always register the RPM. I even tried replacing the wire that runs from the coil to it, thinking it was a short or something. For $10 bucks after coupon and sales, it was hardly worth it. But honestly it only works about 15-20% of the time. It has a mind of it's own. Sometimes I'll start up the rail and it's working great, holding and gaining RPM correctly and smoothly. Other times, it does nothing at all but light up. Another thing I tried was running a diode inline, similar to the way original VDO tachs are wired in. No changes.
I'm running a 009 distro and blue coil.
I fabbed up a gauge cluster and wanted something quick to throw in the hole. I'll eventually upgrade it but for now it looks cool at night. And really, isn't that what's really important