The only issue I have with the S5 is thea stock app, Polaris Office 5. I can't open PDF files which in my case I rarely use.
Compared to the S5 I noticed the performance & smoothness more with ART on the G3, I guess it's because when I first got her I did notice lag. It's only been a few hours since I switched but I'm loving it no issues and was able to open a PDF file.
Also, I noticed when I try performing Software Update I get a message on the S5 saying my OS has been modified and recommends using Samsung Kies which I no luck. Ironically, there's a performance update and I can't update it. I have no problems with the G3.
Galaxy S4 - T-Mobile - Rooted and running Slim Kat 4.4.4
Have been running ART since release and haven't had any issues that I can attribute to ART with absolute certainty. I have had apps crash but they weren't exactly rare on Dalvik either. Battery life does seem to be slightly improved, at least while idle.