As with everything designed by someone else, I am happy with some aspects, annoyed at some, and apathetic about most.
Performance-wise, I have had no issues since installing it. I have 4 machines running it now, ranging from a Mac Pro to a MacBook Air, and everything is working well overall.
Aesthetics-wise, I am on the camp of the people who hate the new look. However, I am also very aware that any look they could have gone for would be hated by some. This is like politics, at any given time at least a third of the nation is going to truly hate the sitting president, regardless of who it is or what party they belong to.
Glitches-wise, I have experienced a few. My Mac Pro has 3 thunderbolt displays attached to it, and I keep getting weird window behavior on some apps. Messages, for example, sometimes does not restore its window at all when it starts up, and some other apps, like all of the new iWork, simply refuse to remember the window position.
When these major releases of software happen I go in it with no expectations. Corporations like Apple make their billions of dollars by largely ignoring rants and displeasure and simply looking at numbers - let me be clear: I do not mean this as a criticism or some deluded sense that consumers should rule the way businesses operate. To me, this model is perfectly logical when you have to release a product to millions upon millions of people. The goal is to make sure it works for most. If some hate it, so be it. If it breaks for some, so be it.
In the end, even those of us who think it looks like it was designed with crayons will probably end up running it on our machines, because the ecosystem requires it. Features we need, upgrades we want, new tech will all require we have the latest version.
I run Windows 8.1 on several machines. This is an OS that I truly detest (I am not a Microsoft hater) but I need to have it and use it because I don't live in a bubble and what I do for a living requires that I use many of the different operating systems that run the world.
Eventually I will get used to the childish look and feel of OS X Yosemite, and the rest of the problems I will also have to adapt to