I absolutely love them I did an experiment a while back. I chose 10 fabrics and cut off a small piece and wrote on them with the frixion pens. I ironed them to remove it and it disappeared on al of the fabrics. I then put them in the freezer and it reappeared on all of the fabrics but lighter. I ironed them again and pinned them to my DH jeans and washed and dried them. I put them in the freezer again and it was very faint on about half of the fabric.
I used a pink frixion pen on dark t-shirt to mark it for applique. When I ironed it to remove the lines it left a faint bleached looking line that washed out. I always test it before I use it on dark fabric in particular. You should really at least test it on scraps of what you are going to use in a quilt. I have always had it iron out but you should always be careful.
I love, love them.