I did it awhile back and my M8 is working as smooth as ever. I also had some concerns but they proved to be nothing. The updates are nice and running the latest has certainly done nothing to harm my M8. I'm actually kind of enjoying the live battery tile actually being accurate for once. Kinda cool. Also the latest update has enabled the data toggle feature that can be placed in the notification pull down for easy access. Not a feature I'd use a lot, but some others might like to have quick access to it.
All in all, I'm glad I tried it. My M8 runs as fast and cool as it always has, no issues at all. Battery life is still excellent.