I have one and love it !! I don't use it a lot but it does make perfect circles if you need them. I'm working on a commissioned project (fellow quiltboard member) who wants to make a Drunkard's Path quilt but didn't want to deal with making those blocks. So, I'm using the circle attachment and stabilizer to make the circle then appliqueing it onto the background block then cutting it into 4 small blocks. Easy-peasy !!
I've also used it on smaller quilts for quilting designs. I say smaller quilts since you will have to move the quilt around in the machine. It works perfect if you're making QAYG blocks.
I want to personally make one like the one Pam Mahshe (spelling?) from Babylock demonstrates on Sewing with Nancy.
It truly is a fun attachment to have if you can come up with uses for it to make it worth buying.