I have and they are my preference to mark my cotton fabrics. They are wonderful because all you do is mark your fabric, sew, iron and the marks are gone. Totally gone. You will not even see a trace of the pen! [See comments posted below. There are some fabrics that do not like these pens. Always test your fabric before using the pens.]
How do I get one, you ask. We sell them on our website now. Just got them in this morning. They are only $4.50 each and come in blue, black or red. You are going to want, no need, one of these and maybe one for your best quilting bud, too.
Go to http://www.beehivequilts.com/notions and scroll down to find them.
Keep on Quilting