I've been running ART since the second day I owned my M8, and I ordered it on launch day. I haven't had problems with apps, at least that I know of. At the time, since my M8 was brand new I figured I didn't have anything to lose with enabling ART.
However, if you prefer to live on the safer side, maybe just wait for the Lollipop upgrade when the newer, better ART is the official runtime.