I just upgraded to Office 2007. Word is OK. Excel adds a few nice features, mainly the 1 million rows limit. But Outlook is just impossible. It is excruciatingly slow. I have a powerful machine, and still, it takes ages to download mail. Even opening an email is a pain. Not only it is slow, it slows down the whole machine.
I googled the problem up, and I found that many users are experiencing the same issues. There are a couple of recommendations available, but none worked for me.
My recommendation: do not upgrade until the issue is solved.