There sure are a lot of complainers on here.
For me, on my late '13 MacBook Pro 13", Yosemite has made everything faster and smoother (well, most of the time - scrolling down in the notifications centre with multiple widgets isn't so smooth but whatever). Every application opens with a certain zip and my fans haven't run at over 4000rpm, even when performing multiple strenuous tasks. On Mavericks, watching a HD YouTube clip would seem to crank up the fans to a audible volume.
I can't be certain whether this is down to Yosemite but since I like the UI and the fact that my MacBook is running smoother overall (and haven't reported any major bugs) I really can't complain.
Same here but a mid 2014 13". Ive actually only ever heard the fans come on once on this machine, and been running Yosemite since the day I got it. Not a single major issue or really anything that would make me want to put Mavericks on it.
People get so bent out of shape over the stupidest thing. Like they don't like a few icons so the whole OS must be junk, and they'd rather use an OS from a decade ago.