These disappearing ink pens are among my favorite tools. I go through several of them every year. I use them to trace around a pattern directly on to fabric for cutting. I also use them to mark stitching lines like darts, fly-fronts, or even quilting lines. It is great for marking match points in the middle of a piece. I rarely make tailor tacks or sew-baste thread markings.
In this picture, you can see how well the line shows up, even on difficult to mark fabrics like blue. This drawn line came from a pen that is nearly dead (I use them until they are dead). It works on most light to medium colored fabrics. It has difficulty with anything that is pink, red and of course black. For those colors I whip out chalk pencils or soap remnants (soap usually works better). I have used these pens on a variety of fabrics from silks, satins, laces, flannels, broacloths, to denims. I use them on personal projects and in industry settings.
The back of the package says to test on fabrics before using. I have had very few problems with the ink not disappearing. If I were to topstitch a fly, I may test it. The last pair of pants I made, the fabric had a finish on it that prevented some of the ink from disappearing. But it all came out in the wash. I wouldn't iron over the ink until it has completely disappeared - the heat/steam from the iron may set it. It would be rare for the ink to become permanent. Most of my ink lines are from tracing around pattern pieces, so they would never be seen anyway. The package states the ink disappears within 24-72 hours. I find it disappears much faster than that, especially when exposed to air. The age of the pen also determines how long the mark will last.
I have difficulty finding this exact pen in the fabric stores. They carry disappearing ink pens, but they don't work as well. For some strange reason fabric stores (the ones near me) carry Dritz marking pens, but not this one. This pen is from Dritz and I special order it from Sew True. Make sure to get the purple disappearing ink pen, and order more than one.