OK, upgraded to Yosemite last night. No problems. Download took an hour or so (5 gb), but the automatic install was about 20 minutes, and so far I've encountered no problems. Don't care that much for the new look, though: I don't like the system font, and everything's flat and simplistic. I understand the graphic design argument against a skeuomorphic interface and somewhat agree with it, but this may have gone too far in the other direction.
Those of you on 10.10 (or on the current iOS), what are your thoughts on the interface?
Those of you on 10.10 (or on the current iOS), what are your thoughts on the interface?