We did a rebuild on a not too old but big site recently. Moved it all to html5. Old browsers be damned!
Google traffic is up but that could be because we played with the on page SEO while we were at it.
The thing that I got out of it is how much I love html5. Even if Google didn't like it I think I am still going to make an effort to move my personal sites over.
It is just so easy and neat! I got a thing for tidiness!