Can do alot with nail polish and get some colors you can't get (or i haven't figured out how to yet) with powder paint. Biggest downfall is the dry time. I used to wait at least an hour - depends what brand - before painting a 2nd coat and would let them sit overnight before tying them.
With the nail polish, you can use any color as a base and put clear/glitter over top to come up with some cool scale looks. Black base with a silver hologram (Brand: Pure Ice) looks good! Some of the nail polish has iradecent (sp?) colors that i don't think you can match with powder.