I just finished the install on mine, and I'm loving it already. Safari is ... well, I won't use that word, but it's blazing fast. I really like the new look. And I like the convenient feedback icon in the dock. Now I've got to start putting the OS through its paces. Woohoo!
EDIT: Curiously, iTunes has exactly the same interface as Mavericks, down to the bubble "stoplights" at the upper left.
EDIT 2: Aha! iTunes was a separate update. But there are still some anomalies. FCPX still has the old interface, as do Motion and Compressor. I had thought that the new look would be automatically applied by the OS. I mean heck, even my old copy of Quicktime 7 has the new interface!
EDIT 3: I'm not seeing the transparency effect. All my windows are completely opaque. (And transparency hasn't been "reduced" in Accessibility). Does the transparency effect happen only on drop-down menus (the only place I slightly notice it)?