A lot of my longarm friends use their TOWAs on every bobbin. I like to do a pull test on my bobbins most of the time. But I still think there is a place for this tool in my studio.
IT is easy to use, click the bobbin in place, wrap the thread under the first wheel, over the second wheel under the little plastic finger and pull toward the slit on left. I normally want my bobbins set between 150 and 170. I like a loose bobbin tension. The slit is actually a thread cutter which is handy. You can even adjust your tension without removing the bobbin case from the gauge.
All in all, it is a very well thought out tool.
I also bought the Superior Top Thread Tension Gauge from Superior Threads when it first came out. I really wanted to use and like this tool.
You attach the top thread to the hook on the bottom and then pull thread using the ring on the top as your grip. It works, but I waste way too much thread and it is kind of quirky. It has always felt awkward and I can affect the reading by changing the speed I am pulling the thread at. I never use this gauge. It is just so much easier to just pull the thread and set the tension by feel.