I joined the beta programme today on Apples website and so far I'm extremely happy with it. Graphics are really smart and everything looks really simplistic - I like the way it's moving. The same occurred with Windows 8.
It's still in Beta (obviously) but I've yet to find a problem. There are a few things that are blanked out (such as changing the view for the albums in iTunes) but everything you need works flawlessly.
Looks amazing on Retina display too! Highly recommended, but really it's just a theme on top of Mavericks. I can't yet take advantage of Continuity as I don't own an iOS device so I can't comment on how that works but I imagine it's very good - a feature I've wanted for a while now. iPhone 6 it is!
Are any of you using it? If so, thoughts?