Just upgraded to yosemite after reading of some poor experiences, and have to say I'm happy with it. I've moved from 10.7 to 10.9 and now to 10.10 each time without any problem. Everything just works as before. The GUI is much nicer in use than the screenshots looked. Everything is cleaner and finder seems much nicer to look at too. It's no more intrusive that 10.9 was with iCloud. Seems like bad press for no good reason for those recommending not to upgrade.