I switched my LG L70 (MS323)'s runtime from Dalvik to ART, after it restarted and said "Android is upgrading..." and it optimized my 64 apps. After it finishes, I get an error message saying something like Unfortunately the system ui has stopped and then my phone restarts and the whole process starts over again. Please help.
ART is experimental at this stage, unless you're on android L (Lollipop) I wouldn't recommend using it.
If you are stuck in ART you can get back to Dalvik by following this thread: http://forum.xda-developers.com/showthread.php?t=2707841
Your device should boot then.
Just do a hard reset on your phone.