I have been running using ART for 2 days now and my Nexus 5 seems to be just as fast if not even faster, especially loading apps. (except tiny deathstar which takes ages now?) and the battery life would seem to have improved as well. My "Ok Google" function does seem to be playing up a bit since the change, but that may be a coincidence. None of my 135 installed apps had any problem being re compiled at boot and so far I havent had any issues. I lost about 400mb of space after the change with 135 apps installed.
lose the task killer...!