I found them about 2-3 years ago at a quilt show, a vendor was marketing them as quilt marking pens. I bought several, tested them, and did some online research. The vendor claimed that you could mark your quilt, then iron after quilting, and the markings would disappear. However, the pen manufacturer noted that if the pen was left in a hot car it could "dry out" or not write, and the way to fix this was to place the pen in the freezer for 15 minutes or so. This made me curious, so I wrote on both paper and fabric, ironed both, and placed them in the freezer. Sure enough, the markings came back. Now, lots of people will say they don't plan on placing their quilts in the freezer, so they don't anticipate the markings to return. But MY concern was if the ink comes back, then it's still in the fabric. At that time, the manufacturer did not have any documentation about the ink being acid-free and/or archival, so I made the choice not to use it. I figure there are several products on the market specifically made for marking quilting patterns on fabric, and I would stick with them. I didn't bother testing the pens before/after washing the fabrics.
Just my opinion and my choice about my quilts. I am positive there will be differing opinions. To each her own.