I don't build anywhere near as many wheels as some folks here. I also don't know near as much about building wheels as some of the folks here. I built quite a few wheels before buying a tensiometer. I bought a TM-1. It wasn't the cheap side of me coming out, but I just couldn't see the benefit in spending more. It's only to check what my fingers and ears are telling me. After I realized that the "same tone" in every wheel/spoke combination didn't necessarily produce the same tension- I needed some measuring device to test the work and get closer to the target number (whatever that is.) The TM-1 will tell me what I need to know as well as any other instrument currently available.
What is disturbing are the reports on the need to "recalibrate." I lost my Park instruction sheet but I recall somewhere in there about the cost of sending it back for re-calibration. What? I'm not gonna fiddle with the initial factory settings, and I can't see the spring relaxing enough to throw deflection readings off. To check this occasionally I've built a little fixture with spokes having a known deflection using the TM-1 when it was new. It's simple to test that the tool continues showing the same deflection as originally witnessed.
So... Every tensiometer I see out there uses a spring to "deflect" the spoke. That spring isn't under huge tension at any time, and it doesn't cycle with the frequency that will cause a "weakening" of the spring, so what gives? It should never need recalibration if designed properly. Maybe someone needs to re-think how spoke tensiometers are built.