How can I tell if it would be safe?
I am running ViperOne 7.0.1 on an HTC One m7.
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On my Xperia M, running the latest nightly CM, it always causes trouble (every 10 seconds an app crashes), so I occasionally try it (Is it working yet? - No, well, I guess it's time to switch back), but I never use it - others say it's working fine for them. If you are able to switch to ART, I assume you are rooted and have a custom ROM installed, so you are also able to make a full backup from the recovery. You can try it, but make a Nandroid backup first, as it could screw up everything to the point you won't be able to switch back from inside Android. I'll say this again: switching to ART is NOT safe! Always do a full backup from the recovery before you try it!
On the first boot with ART, it's going to take a lot of time to compile everything, be patient. Good luck!