I have one. Didn't make it myself, though. It actually works surprisingly well. With the right material in the tip of the plunger you get a nice coal with one good "slap" or two of the handle. Gotta make sure the gasket / seal is good and well lubricated, and char-cloth is a little too delicate. Tinder fungus seems about best to me.
It's fun to play with, but anyone with experience using a good flint & steel set can get a coal as quickly, and building the fire from there is the same. And flint & steel is PC, so there's that.
If you decide to build one, have fun. If you'd like to buy one, let me know and I'll try to find the eMail of the guy I got mine from. He was really helpful and actually sent me a better piston and more supplies than I paid for.