I've been tempted for quite a while to do this as I think it could look really cool in the right car..... Not sure mines the right car though.. Lol. I think it'd really suit my wheels, but maybe not the car.
The other thing is it will depend on the tyre as well. Bridgestone will look cool, toyo etc, but my recent addition of mpss I don't think it would work so well.
FYI I've also read that a tile grout pen actually lasts longer than the paint pens.