Few things about ART:
1. It's still VERY experimental and you probably shouldn't rely on it for daily use. There is a chance that instability can rear it's ugly head at any moment. It's there for developer use right now.
2. Some apps will constantly crash with it and you might even have issues trying to install those same apps again once you switch back to Dalvik. Manual fixes or a factory reset will resolve these.
3. The GAPPS packages in Custom ROMs don't currently play well with ART right now and pretty much all just crash over and over again.
4. This is probably the most detailed write up about ART right now: http://www.androidpolice.com/2013/11...uts-in-kitkat/