I'm in the same boat too with my Mac Pro 2009 with Mavericks. I've been holding off upgrading for some time.
The only reason I'd upgrade is to run MacVidCards' Nvidia GTX 980.
I also hear you need to do a bit of fiddling if you want TRIM Enabler to work with SSDs on Yosemite. And there have been some (unconfirmed) reports of Yosemite having issues with some interface cards like USB 3.0 and eSata ones.
I dual boot yosemite and win81pro on my 2012 macpro, and am very happy with yosemite. No complaints about it.
I do have one strange behavior in that anytime I switch from os x to windows, my mac pro stop chiming at boot. It will not chime anymore after that unless i reset the pram. If I reset pram, with only the efi win81pro disk in, it will chime and boot into windows with no problem. If I plug in the os x disk and boot into that, then boot again back into window the chime stops.