I took my 2008 iMac from 10.8 to 10.10. I'm elated. The best OS since 10.6, with time it will be better. Also running an ssd. Everything feels quicker.
Did a fresh install. First time since 10.6 that all the stuff I actively use (remote management, screen sharing, file sharing) worked without fussing around. The most annoying thing I've seen so far is the Finder window size is not sticky on mine.
I'm not sure I use any "new" features. If anything, I'm "de-featuring" as I find most of Apple's new stuff is either unreliable, is crippled or is another annoyance.
Spend time making sure your apps will support Yosemite. I was surprised how many latest version apps would not install or installed then I got a message they would not run under Yosemite. I'm probably running with 15 fewer apps than I was. Nothing critical fortunately. If you are a DropBox user I'm finding its frequently using more than 50% of CPU even when its doing nothing. A lot of people have reported this. I quit DB after it does its sync. Assume it will be fixed.