If you'd search the forums you'd know that many users have had various issues with Windows 7 and some of which has made some users go to using Parallels while others haven't had any issue that would make then stop using Fusion.
My suggestion would be to give both VMware Fusion 3 and Parallels Desktop 5 a test drive as both offer free evaluation licenses and then install Windows 7 in both without activating it, as you don't have to for 30 days, and see which product works best for you.
