I've tried it this morning with the PC version. So far it seems to be working. I'd advise backing up your iTunes library files before you upgrade, just in case you run into any issues and have to revert back to version 10. I wouldn't expect problems, but I only spent 10 minutes or so with it.
I've not updated the Mac yet, so I've yet to test the Mac version.
The new UI looks like it'll take a bit of getting used to, but once I turned on the sidebar everything got a lot more familiar. Most of the functionality looks like it's still there, it's just not immediately obvious at first glance where everything went.