As an ex-open-source-evangelist, and someone who's used both, I'm positive that Office 2007 is a much better product. It's lighter, faster, prettier, more user-friendly, and more widely accepted. If you can get a discount on Office through an employer or university, I don't see any reason to use OOo.
EDIT: Of course, if you have moral objections to buying from Microsoft, that's another matter. Personally, I think they've been somewhat more ethical in recent years, certainly nowhere near as bad as they used to be.