iTunes 11 has been released for OS X (and Windows)! Check Software Update to get the new version. Or visit Apple here for the iTunes 11 file.
Initial impression... Wonderful and quite speedy! I love the new UI. It's so much easier finding music to play now.
I did find two glaring problems though...
1. I am in the process of digitizing all of my CDs into ALAC and AAC files. From what I can see there is no way to easily number tracks ripped from my CDs like in iTunes 10.7. i.e. no Previous and Next buttons at the bottom left of the Get Info window. I have to number each song individually, which is going to be a huge pain as I digitize the rest of my CDs. In previous version of iTunes you could right-click on a song and then proceed to number all your songs without leaving the Get Info window. See the screenshots below.
EDIT: You do get the Previous and Next buttons by right-clicking on a song in Songs and Albums views. Definitely not in Artists view though.
2. There are some rendering problems. See the screenshots below.
I'm in the process of writing a huge review of iTunes 11. Stay tuned... :)