well, I played with the LG G3 for about a week before moving to ART. Once I moved to ART, I did notice less lag on things such as opening apps, moving about in the app folder and even on the entry of my unlock pin on the lock screen. Is it going to blow your mind with the difference? No. But I can tell some differences with the smoothness and to me, that's worth it. However, I haven't really noticed any difference in battery life one way or the other.
Really interested to see how ART will impact things once it's full-blown in the next version of Android.
One additional note: I attempted to use ART on my HTC One M8 and reverted back after a week as it tended to have the opposite effect. I started to see more lag. Was it due to ART or some other app that I had recently installed or some other factor? Not sure, but I had to revert.