Yes, I have been using Bobbin Genies for several years, in my Gammill and now my Viking Mega quilter (which I use for piecing quilts)
They are to replace the back lash spring that is in your bobbin case.
I love them so much I purchased several dozen, and Oh, you don't need that many in a life time. They last and last.
I use the same one in both M and L size bobins.
Sharon Schamberger (think) is the one that I saw them on.
Keeps your bobbin from spinning to fast when it needs to stop, hence a birds nest.