I'd suggest trying out the beta on a computer you don't really use/need. Don't install it on a computer that you regularly use/have important files on, because it's just too buggy for use with a computer you use regularly D:
EDIT: This
site seems to say that a lot of people have no problems using CS4 with the new OS, but there are also people posting who either can't install it properly or have it crash after 10-15 minutes.