After having Yosemite from beta on my rMBP 13 and then on my MBA 11 I've gone back to Mavericks. I update my Mac on a yearly cycle and usually keep the OS current, first time I haven't done this. Fact is there was nothing new in Yosemite that I needed or wanted, possibly iCloud Drive, but then that didn't work properly -- so back to dropbox. With Mavericks I have a quieter, faster, more stable machine. Less RAM being used, better battery life, and it's easier on the eyes -- in terms of eye strain. Every Mac OS I've had from OS 9 has felt like an improvement, this is the first time it hasn't. I hope by OS 10.10.5 they've sorted it out. I'm actually holding off upgrading my machine this year as I don't want a Mac shipped with Yosemite. Very easy to downgrade: cmd - alt - R held at start-up, into Internet Recovery, Mavericks automatically put back on. I'm saddened that Yosemite is not better than Mavericks...for me.