I love them. I use them to mark embroidery patterns and then I'm sure to cover them well. Once at the beginning of the summer I had my project in the car, and it got so hot that the marks all disappeared! I had to wait til I got home and put it in the freezer before I could continue working on it. Once you learn how the Frixion behaves, you'll know better how to use it. It's really great for some things, not for everything.
When I'm going to want markings to be removed, I try to use white tailor's chalk and then brush it out afterwards.