I'm new to embroidery and began it as a craft to do with my little girl. This pen shows up as blue marker on fabric, but when exposed to water, the blue instantly washes away and disappears. It has come in handy for a variety of other fabric projects since I purchased it. For example, we made fabric rosettes by cutting circles of fabric and stitching them together. It was so easy to use this pen to trace the circles and convenient to know that it would rinse away and not show afterward.
I can't comment on how long the pen will last, but having had it a few months, I've gotten much use out of it. It's a great crafter's tool!