But how much of a difference does ART really make over Dalvik? Someone over at Reddit shot a video showing the difference on a Nexus 7 (2013) running the same AOSP based KitKat ROM with the same amount of apps installed.
Since ART is a native runtime, it will significantly help in reducing the loading times of applications while burning through less CPU cycles. There should also be an overall improvement in app performance and switching between various apps. The above video shows exactly that with the ART enabled Nexus 7 consistently loading Photoshop Touch by more than a second faster. Keep in mind that there are no other changes between the two devices, so it is pretty significant that users can see significantly shorter loading times with just the switch to ART runtime.
I have been using the ART runtime on my Nexus 4 running an AOSP KitKat based ROM, and the difference it makes is pretty significant in day-to-day usage. If and when Google decides to replace Dalvik with ART , all the lag issues that still plague Android will be solved once and for all.