Started by buying one of these to try out, and promptly proceeded to buy four more. They are great! Due to the small size of the nib, you can use thinner paints in these than your average steel tip. I've successfully used these with the obvious candidates such as Ruff Stuff Paint/thick enamel lacquer, but also with MTN Street Paint and even Coversall INK!
Nice, easily squeezable body and neatly fits in your pocket. A must-have if you have a lot of rusty or rugged surfaces around just waiting for you to sign them ;)