i had one of the blue water-disappearing pens ruin a beautiful old pink and white quilt, every since then i will not use anything except pencil and chalk, and i test every time. i try to not have to mark my fabrics, if i am doing applique or hand piecing i mark at 1/8" instead of 1/4" then i know i have to stitch exactly 1/8" inside my mark. i still have the line to follow but if the pencil is not going to come out it is not going to be visible. those new pens are cool looking and if i see one somewhere i will probably buy it and try it. i do not use them ever on my quilts but i will try it for something else, like a table runner or something.