If you own the Nexus 5, Nexus 4, Nexus 7 or Nexus 10, and they are powered by Android 4.4.2, you can enable ART to run on your device and have a fairly smooth ride. ART wasn't ready for prime time on Android 4.4, but the later build seems to be offering a better experience.
Let's get going. First, you need to enable Developer Options on your Nexus device. Go to Settings > About and find the build number. Tap it repeatedly. You'll be prompted when you are close to enabling Developer Options, and then you will see the final prompt.
Now, go to Developer options and scroll down to "Select runtime." Pick ART and it is all over but the shouting. Your Nexus device will reboot which should take at least 20 minutes and all of your apps will be converted to ART. Keep in mind that not all apps have been configured to work with ART.
The last step is to let us know if you notice the difference in speed when opening and running apps on your Nexus device. Eventually, all Android devices will be using ART.