I had the same problem but it is now fixed.
I'm afraid the only solution would be to get rid of the stupid itunes 12.0 and install itunes 11.4 again. But there is a glitch if you don't do it right.
Don't worry though it is easy.
You have to follow the tutorial here: http://www.pcadvisor.co.uk/news/audio/3415214/how-downgrade-itunes-11-itunes-10- in-mac-os-x/
It works the same if you try downgrading from 12 to 11.
I just did it and I wanted to share this because I am now relieved ! My itunes is back as it was before the update.
You can get a copy of itunes 11.4 directly from Apple here: iTunes 11.4 for OS X 10.6 (it works for 10.6 and higher)
The only problem is to get a copy of Pacifist (it is NOT free), but you can find one free VERY easily if you use torrent (you see what I mean).
If you don't want to use torrents, I can give you mine if you want, maybe I can find a way to share it on google drive or dropbox...
And don't forget to back up the itunes media folder! It is very very important indeed. I used a Time machine backup of the itunes folder before i upgraded to 12 but I think it should work for any itunes library.
Tell me what you think.
You can't install itunes normally (through the dog file without pacifist) because it will say that it wan't installed properly