ART actually comes as an option built right into Android 4.4 and speeds up the time that the operating system uses to read application code significantly. Preliminary tests show that app execution time, which is everything from launch and operation to occupying processor resources, is cut in half for most of them.
This enormous speed gain comes with caveats, such as larger apps hence longer installation times, but the ART experiment seems ripe to replace Dalvik in some of the next major Android versions. It replaces the just-in-time (JIT) real time compiling with ahead-of-time (AOT) one during the app installation, thus cutting down on app launch times a great deal, which is especially useful on devices with limited resources.
In fact, you can test drive ART right now, even if you aren't an Android developer - go to Settings>Developer options on your Nexus 5 and enable it, then reboot. It will take quite a lot of time until your apps get recompiled with the new runtime, and you probably won't notice any difference at this point, but peeping into the Android future might be worth it.