I think they look slick. I'm putting them on all my lights and the trunk panel on sunday. I don't want to paint stripes on the car, I think the black on white is going to break it up nicely and look great with my limo-tinted windows. I'll post pictures once it's done. I kinda like the overlays, but they're not black enough for my taste.
I dunno how they're gonna look on a black car. I saw an all black camaro the other day including the black light covers. It kinda looked like a blackhole, and not in a good way.
The great thing about mustang drivers though is their willingness to customize. Mod it out, dress it up, make it yours. The opinions of others should matter little.