The TM-1 is a must-have tool for keeping spokes properly and uniformly tensioned. It accurately and reliably measures the absolute tension of each of the spokes in a wheel, as well as the relative tension between all the spokes in a wheel. The TM-1 works on nearly any bicycle spoke??_no matter what the diameter, material, or shape. Not just for the experienced mechanic, the TM-1 is for anyone building or truing wheels, diagnosing wheel problems, or assembling new bikes.
Strengths: Industry Standard.
Calibration is possible.
Weaknesses: Translation between gauge scale and actual KGF chart.
Requires some finesse to get a repeatable result.
Price as compares to other products on the market.
Full disclosure: I am the author of the iPhone Spoke Tensioner (http://itunes.apple.com/us/app/spoke-tension-gauge/id518870820?mt=8)
For years I used this as my only tool for checking tension, and it has done an admirable job and I have built a number of wheel sets as well as true up wheels with it.
There are several shortcoming though. First is that I find it tiresome to alternate between the spoke wrench and the meter constantly. Similarly threading the tool in between the spokes and getting on the narrow part of a double butted spoke is somewhat time consuming.
Second, when comparing with two other identical TM-1 tension meters I found discrepancies of up to 20kgf. Most measurements were within an acceptable range of +- 10kgf but 20kgf could be the difference between a good build and a cracked rim.
Lastly, the repeatability is difficult until you have used the tool for a while. Even with the same tool I would get some smaller (10kgf) variations in tension for the same spoke. This depended on how quickly I let the tool clamp on the spoke, how long it sat there and in some cases the angle that it was on the spoke.
Overall, a good tool but for a serious wheel builder but very expensive compared to the competition like the iPhone based spoke tension meters.