I'll probably jump on the band wagon at 10.10.5
It's been real entertainment to read these forums, seeing the angst and pain everyone suffers going through the .0 to .4 iterations.
I got burned with Leopard (10.5) - I installed it on day-one, and spent 4 hours with Apple Help line trying to salvage my photos from the dead Mac.
With Snow Leopard -- which everyone reveres as rock solid -- they forget that Snow Leopard, for me at least, didn't really get rock solid with Apple Mail until 10.6.6, and attained its legendary status for stability by 10.6.9. I installed SL at 10.6.4 and had some teething problems until it got to 10.6.6
So for Lion I waited it out till 10.7.4 and it was rather trouble free. I watched from the sidelines as people tore their hair out sorting out bugs in the .0 to .3 releases.
For Mountain Lion, same. I jumped in at 10.8.4 when things were fairly good. Right now, ML for me is rock solid.
For Mavericks 10.9, I never installed. Same as ever, there were claims it was stable from Gold Master, but the inevitable bugs in Apple Mail only became widespread when it went public. From reading reports, I'm not convinced Apple Mail in Mavericks 10.9.5 has been totally fixed for Gmail IMAP. Notionally, I would be willing to take a risk with Mavericks if its features were compelling, but there is nothing in Mavericks that attracts me.
I think I'll wait it out till 10.10.5 with Yosemite, and then come onboard. I think Yosemite looks cool.
Even if OSX is free, it costs me to upgrade because I have to pay for the latest update to VMWare Fusion, plus the possibility of having to re-install Windows 7 each time. So unless the OSX new features are really compelling, I tend to stay put for a while.