I got one after a long struggle with getting tension correct. I discovered I had several bobbins that caused erratic tension, and after tossing them I have much less problem. My machine likes the bobbin tension around 180-200, and then I fine tune things with the upper tension. I also got a digital readout for my upper tension. I don't switch threads around very often, but when I do, that is the first thing I set.
I believe that most of my tension problems at the start were due to the bad bobbins. They were a bit heavier and are what came with the machine, but when I bought more, they were a different style. It wasn't evident with the drop test that the tension fluctuated, but with the towa guage, no matter how carefully I pulled, I'd see the gauge bouncing. Because of that, I have no confidence I can set tension correctly by using the drop test so I don't try anymore.