Yes, I have used them A LOT. BEWARE: that ink is in there forever. Granted, ironing causes it to become transparent but it is still there and should the quilt be i a cold place, the marks will return. There is nothing that will take that ink out!!!!!!!!!!!!!!!!!! I still use Frixion for certain types of marking but I attach a label to the quilt telling the owner that if the quilt becomes cold, the only way to cause the marks to disappear again is to use a hot iron.
If you use a scrap to test the Frixion pen, it may wash out with stain treatment solutions but be warned, once that ink is ironed to cause transparency, there is nothing, and I do mean nothing, that will remove it!!