Has anyone done this yet? I'm reading how it helps speed things up over using Dalvik. Still don't really know what it does. Just did it on my Gnex running Shiny's 4.4 ROM.
To do it, head into Developer Options and swap over Dalvik to ART. Some are reporting FC's though so tread softly.
Edit: Heads up, if you have a lot of apps and you enable it, the reboot takes a LONG time. It's not bootlooping, just swapping all the cache over.
2nd Edit: Some reading here:
(FYI, I have 172 apps and it's coming up on 10min to optimize them and still not done)