I tried ART when we first got KitKat and at that time there were still some apps that had incompatibilities so I switched back to Dalvik. Since the L announcement I went ahead and enabled ART again and have had no problems whatsoever. No problems with any of the 50+ apps I have installed. Perhaps it's placebo, but the phone sure seems a lot snappier to me. Battery life seems comparable. Even if it is placebo, as long as the phone feels faster to me, why not use it? You can always just switch back if you don't like it or have some apps that don't play nicely yet.