I switched to using the Twenty Twelve WordPress theme a few days ago. I really am liking it although I’m not sure about the header image being below Site Title and Tag Line. That looks a little strange to me.
Having said that, I think Twenty Twelve looks fantastic on the demo site.
Currently you have get Twenty Twelve from the WordPress trac repository as as far as I know it is not released yet. They will release it as a stand alone theme soon I believe, so you can try it out before WordPress 3.5 is released.
I did a
git clone of the WordPress code mirror on github, into my home directory. Then I symlinked the
~/wordpress-git/wp-content/themes/twentytwelve directory to my live WordPress install path. That way I can simply do a
git pull to update the theme.