my review is that i changed to art from dalvik on the second week of owning this s5 (all stock, nonrooted) last april. everything was fine and stable then. It felt really speedy because i had just moved from a note 1 to this. I also had experienced no FC's or any kind of instability. I only use this phone for the basic stuff, camera, plus touchdown. I thought all was normal and i was 'living with' the camera lag as I thought it was just flaky advertising that it was speedy with 3 second camera focusing.
Then, sometime in the past 3 weeks, the phone would heat up, occasionally hang, and drain its battery like there was no tomorrow. That was only recoverable using a battery pull. It was getting to me so i tried to find the error and installed OS Monitor
to see if there was some background process eating up any resources. I had found a recurring process called kswapd0 coming up every now and then eating up more than 20% CPU time.
I find that strange as android is built not to need swap which is what this kernel process is for. so, I tried google-ing around for errors related to the kswapd0 on the S5 even in this forum but found it for other phones only.
After going through this thread, It occured to me to try to get out of ART and go back to DALVIK and i tried that just now. Bam. Phone feels and behaves snappier. Even the camera is faster loading, focusing, and taking photos.
Will be testing this more for the next 'as long as it feels stable' and report back anything.
so far, conclusion is: stay with Dalvik!