I love the look of smoked headlights and taillights on black vehicles.
My issue with them is visibility... ESPECIALLY on a bike. I have tinted signals and will do the smoked taillight on the Connie. I've tried to counteract the smoked signals with high output LEDs. It seems to be pretty visible in bright daylight. I'm also lookign at Hyperlites or sim to supplement the rear of the bike. The headlight is different, as there is no good way to really increase light output. HID conversion might counteract the tint, but then why tint in the first place?
On a car I would have less issue, because you could use brighter bulbs for daytime (signals) or nightime (headlights) driving, while having the blackout effect for the main headlights. Then again, I always have amber driving lights on my cars, day or night... so there's still some visibility from the front.
+1000 on the illegality of it. Check your local laws, but I think it's one of those things that are illegal in most states.