The main change is to the 'look'. The GUI has a darker colouration and now sports the Apple software look. Where things are located has changed, with some menu item reshuffles. There are also some name changes. Overall it is not a major change but there are a lot of little differences that mean playing the "where's the ____ gone?" game.
Already the reviews are coming in so I won't bother here (well maybe after I have plumbed the depths of it). Here is one from a user who has been with Logic since version 1 (I'm a newbie - I joined in at version 3). And here is an article about some of the hidden features. eMusician have reviewed it here (along with a video tour).
Version 10.0.1 has just come out so there is no lack of support from Apple.
The documents covering what's new are here and the manuals are now online.