I've had the new Dior Rock Coat Top Coat in my possession for about a month now. I've been putting it on top of every single nail polish I've worn in that month and it's all looked like crap. I can't get it to apply evenly without looking steaky. With one stroke application it doesn't cover the whole nail (except on the pinkie) and with 2 strokes it pools and streaks... It's making all of the colored polishes look like mud...
Finally today I put it on top of Essie Mochacino and it actually applied well. I think it's because Mochacino is the first darker hued polish I've tried it on.
I adore the concept. I think it would be AWESOME to smoke out any polish. But to be honest it's just not worth the money. I expected more out of a top notch brand like Dior. (In their defense, Dior does state that it works best over darker colors. I think it would probably look good with a glitter polish as well.)
Dior Rock Coat retails for $19.50 and at first quick search appears to be sold out online. You can try calling counters at finer department stores if you want to hunt it down, but it's likely another designer will also release something similar and hopefully better.