One thing I would watch out for is that if you use Master Pages then you may end up duplicating them for both view engines. I had a good ol' WebForms Master Page, used by all my regular .aspx content files, but creating a new view using Razor means I have to use a duplicate Razor layout page alongside that old Master Page.
I haven't looked into it too much, but at first try I get 'The file "~/Views/Shared/Site.Master" could not be rendered, because it does not exist or is not a valid page.' and certainly there's nothing in Scott Gu's blog entry introducing Razor to suggest you can combine them, but I could be all wrong here.