I've had Apple's latest big cat loaded now for one week, and I have to say that I've yet to have any real problem with it. While I know others have had nightmares upgrading their machines and are less than impressed (my experience a year ago), mine went off without a hitch. After using it for the last seven days, I can say the word that best describes it is snappier. Everything about it is faster, and I see a level of refinement that was sadly lacking in Lion.
The one feature I like the most is the Notification Center. Granted, I don't get a whole lot of email these days, but it's nice to see the emails pop up when they do arrive à la the notifications in Outlook on Windows. I also think Power Nap is pretty cool, although its function was something I erroneously believed had already been a part of the OS prior to this.
Another thing I really appreciate is the Twitter integration. I tweet a lot, so it's pretty cool that it can be done from so many places within the OS.
Wired has a good rundown on some of the lesser-known features here and I highly recommend checking it out.