I wrote this Friday night, after a friend asked the same question:
So far, awesome. Very elegant refinements in the UI, and the work under-the-hood is noticeable in the overall smoothness and responsiveness. Orders of magnitude better rev-point-zero than 10.5 was.
I haven't checked out all the "goodies" yet, but I'm not rushed for time

One thing I did play with was the screen recording feature in QT-X. It works very well - it even mixed my voiceover with an on-going audio chat. It'll be a very useful help aid and for some, worth $30 itself.
I'm getting used to the changes to Exposé. They changed the layout a bit, but added the ability to use the Dock and you can now get directly to an app-window-only Exposé from the app's Dock icon.
On the footprint side, my MBP gained about 11.2GB (base 10 - get used to it), our MBA gained 8.1GB. I suspect the difference might be because I had already weeded out a couple of GB in printer drivers on that machine prior to installing SL. The elimination of unneeded printer drivers is responsible for good part of the smaller footprint.
Since I wrote that, I've had almost two more days experience with it and I still have the same impression. A lot of the little, rough edges in 10.5's UI have been smoothed out. The speed improvement is very noticeable in launching apps, the Finder, Mail, Safari (yes, "snappier"), and just a general responsiveness. We are already seeing some of the fruits of the massive labor which has been accomplished under-the-hood, and we'll only see more of it in the months and years to come.
Some reviewers have been disappointed by the lack of UI/noticeable changes. Overall, I have a different take: the fact the user
doesn't have a new OS "in their face" is a good thing! There is no reason to make changes for the sake of change. Apple (as they've said all along) has put forth enormous effort in developing the operating system itself - something a lot of users don't have an in-depth knowledge or understanding thereof; or, for that matter, even care about. It's no wonder there are so many dreary, "I don't see anything new" reviews. To those reviewers, I say go back and re-read the sales brochure. Apple has
more than delivered as promised. To see the very few glitches in a point-zero rev is a testament to all the hard work which has gone into both Leopard AND Snow Leopard.
Good job Apple!