So I've been running this OS most of the day, and so far, I'm mixed on my like for it. It seems like so little has changed, and what has changed, I have on my iPad/iPhone. We now get unified messaging (no more iChat), and a lot of iCloud-type features, like shared notes, reminders, and messages. If you subscribe to the iOS eco-system, this seems to work well (just like it did in SL).
The new default wallpaper looks a lot like the one we got previously (the space galaxy shot).
We do get a new version of Mail, that so far looks like the old version, except the animations are all broken so stuff happens lightning fast (which I like, haha). Mail also seems to index every single freaking mailbox you have, which took forever to setup, but after it was done, it was kinda nice cause it groups conversations regardless of what folder they are in (including your sent messages).
Parallels Desktop is broken badly. Even the 32-bit hack that people are posting doesn't seem to work for me. Then again, all desktop visualization breaks every single time they update the OS, and this is a preview.
The updates come through the App Store. Safari seems very much the same.
Messages and the iPad/iPhone Apps function much like you'd expect them, and updates happen pretty quickly to my iPad and iPhone. It was pretty interesting to see that my iPad games from Game Center weren't visible on my Macbook or iMac though. I guess they are different apps... I guess.
More as I keep using it!