I have been building wheels for 25 years without the Park TM-1. I have built and re-built wheels for my two road bikes, and have built probably 20 wheels. I have never built a rear wheel that hasn't required re-truing after two or three rides, until using the Park TM-1 tool. I wish I had purchased this tool 25 years ago! I rebuilt the rear wheel for my road bike and tensioned the spokes according to the tension chart provided with the tool. I have not needed to re-true the wheel at all since checking and balancing the tension in the spokes - and that was two months and more than a dozen rides ago!
New wheels, whether machine or hand trued, may arrive with incorrect spoke tensions which can lead to unhappiness. Checking before use is good, and this also takes some of the mystery out of replacing broken spokes on old wheels. Even if you only have one (but why stop there?) set of wheels to maintain, this can still tell you valuable info.
The gauge is spendy but worth it. I used to build wheels and true them by feel. But now that I have this gauge I won't go back. My new approach to an existing wheel is to run through the wheel and get an average reading before I do any truing, then go back through and use small bits of tape to ID spokes that fall way outside the norm.... low tension get tagged down at the hub, high tension get tagged out by the rim. Then when you start working out the issues it becomes really obvious what you need to do.
For new wheels get 90% of the tension on the spokes, set the spoke heads in the hub whatever way you normally do it (hammer, flexing pairs, stepping on the wheel!) then go through and even out all the tensions before doing the last 10%. It will speed things up a lot and you end up with a much stabler finished product.
My only word of caution is to always measure the spokes the same distance from the rim, and release the gauge slowly and smoothly. There is friction in the gauge and if you pop the handle open you will get readings that do not repeat because things are too dynamic.... just be gentle and consistent and the readings will repeat accurately.
I had been using a friends tension meter and didn't understand how to use it properly. At the risk of using my manhood card, I read the instruction and discovered that my wheels where in better tune that previously believed. The meter comes with a spoke gauge and convertion meter to tension chart which are estential. So is the release motion.
I bought the TM-1 tension meter to assist in building up a pair of wood rimmed wheels for a retro bike project. The tool was invaluable in assuring that tensions were uniform and as a result the wheels have been solid. This project used steel butted DT spokes. I also used the tool on a Mavic Cosmic Ultimate tubular rear wheel to bring the wheel back into true w even tensions on the flat bladed spokes - again worked like a charm. Wish I'd had something like this when I was building lots of wheels back in the '70s and '80s - its simple to use and cheap compared to what else is out there to do the same thing.