It's very stable for me I have Yosemite on a partition but I've only been back to my Mavericks partition when I really had to. Yosemite is working a whole lot better. I'm gonna keep it this way though, this way I'm sure it's safe
I have been running it as my main OS since PB 1 too, and I have had no major issues with it. Some minor bugs, lockups, and 1 Kernel Panic are the only thing I have encountered. Not bad at all.
That being said, PB 3 seems very stable, but as always make sure you backup your current set up immediately before installing, in case there are any major problems.
Backup your data and go for it. I did it three days ago and oh boy, I don't want to use mavericks never again. This thing is fast, beautiful and very useful.