I've played a few games now with this mastery, and it is a great addition. You have a nice little particle to show that it went off, it clearly tells you how much damage it's adding, and you can make use of it all game long. In lane, it highly encourages good trading patterns, and later it feels like it adds substantial damage to your combos.
On Lux in particular this is an amazing mastery. In lane, Q+auto+E+auto trades are hyper efficient, auto+E+auto trades are also amazing. And the mastery adds to you catch/ burst combo as well later in the game- Q+E+R. Really pleased with this mastery.
All the "cunning' keystones actually seem really nice IMO, now that I think of it.