Some first impressions on my 2012 Macbook Air:
First, it looks as beautiful as ever. Though they did change the color of my side dock from black to the same color as the iOS dock, which I don't like there either.
It also seems slightly slower than Mountain Lion. The boot time and wake up feels a second or two longer, and apps like Firefox take a little bit longer to open as well. It's nothing huge, but it is noticeable. Might try doing a clean install in a week or two when I'm less swamped with school work, see if that speeds it up.
The battery life improvements are very noticeable though. Got through an hour and a half class starting with 19% battery, which is outstanding.
I also couldn't care less about the new apps. Why do I need Maps on my laptop? I'll never open any of them.
Overall I'm not crazy about it, but it's not awful either. Some good, some bad.