Just wanted to say to be very careful where you keep your pens. I had some in my sewing room, and didn't realize I had other pens that looked very similar and ended up marking my quilt with a normal pen!! NOT a good thing. BTW, this wasn't my own testing, but a blog I found. :)
I believe I will probably use the frixon pens for simple wall hangings, table runners, etc., but not for hand-quilted bed quilts that take a very long time to quilt, and ESPECIALLY not for anything of heirloom quality. I just don't want to take the chance.