This is the one reason why I keep bringing my Mac Book Pro to work every day. I'm the only person in my organization who uses linux (Ubuntu on a desktop) and Openoffice.
Openoffice usually mangles a word or excel document I try to open, so I have to use my mac version of MS word/excel. But this isn't because MS word/excel is better (at least for a casual user like myself...I find that creating documents with OO and converting them to PDF to send to my colleagues works great), it's because MS probably works at keeping the two incompatible.
I tried Open Office v3 and would say that it's at about 90-95% of the functionality and quality of MS Office. Unfortunately, the 5-10% includes a few obscure features on both Word and Excel that I use frequently, but I think for a lot of normal people, they'd be just fine.