I have at least 15 machines and have never had the need for any product like that.. my machines and my stitching are great.

I'm with Candace on this. I've never had a bird's nest or eyelash so there's no need for one.

They are for fmqing and long arms.

I made my own out of silicone oven liner. I don't know if they help or not, but I haven't done much FMQ.

I have one in my Baby Lock Quilter's Dream and love it. Haven't had any "problems."
I started using my Singer for piecing recently and forgot to use the genie. I noticed nests on my sewing; using the same thread as in my Baby Lock!
They must be doing something!
I have read that they last for a LONG time. I'm still using my first one. Just don't lose it when you change bobbins! They are tricky to find on the floor.

I use to use them in my Viking Designer SE just when it was set up to embroider - really made a difference! Then I tried them for all sewing in both my machines and I do feel there is a definite difference in the stitch quality especially when FMQing. I believe the quilting diva Sharon Schamber also uses the Genies. These may be just a specialty type of washer that might can be had via a local plumbing store (I'd heard this on another group but have not checked it out).

My LQS let my "try these out" during a class and I did not notice any difference.

There are 2 sizes they should be marked for domestic sewing machines (I believe 6942A) and there is a size that says for Long Arm machines. You have to be careful about that as the ones marked for LA are only for the LAs that have the larger bobbin - LAs with the regular size bobbin use the ones for DSM

Forgot to mention, these are not necessary for regular (straight) sewing but are intended to decrease the backlash that you get when FMQ. I do find them helpful for this

I agree with Sunnye. They have helped me with my regular sewing and also with my quilting. Although I haven't gotten up the nerve to do FMQ, yet. That is on my to do list, when I don't have anyone else in the house to hear my comments to myself when I am not fast enough to keep up with the machine.