I tried it on a test piece and the lines came back once they got cold even after the wash. Not sure I would ever use them.
Cindy Needham swears by them but she gives a disclaimer: She ONLY uses them on her own quilts. Sometimes the pen can leave a faint shadow line, so always pre-test. She only uses black and blue as she's had problems with the red and orange. She also recommends marking them so you can easily distinguish them from a regular pen.