Never used the FriXion pens for marking quilt patterns, but I have seen plenty of discussion on Quilting forum. People often regret using them because the markings reappear when the quilt gets cold.
FancymomYou will love them for quilting. The marks disappear with the touch of an iron and it has to get pretty darn cold for the mark to reappear...like freezer cold. Most of the marks are on the back of the fabric for sewing or trimming.