In my opinion, Microsoft Office is more polished and you will tend to get better compatibility opening Microsoft Office documents (especially the newer Office Open XML format) in the same software used to create the document. Of course, if you're constrained by budget, Org offers great performance.
I use OO just because it runs natively on linux, but I must admit that I do prefer MS Office, as sometimes OO can be a real pain to try to get it to do what you want it to.
I have a strong preference for MS Office. However, if I was short on cash, and needed a solution there is nothing wrong with using OpenOffice. I just don't think the quality is as good as MS Office. You also have to consider Google's Documents platform. It works pretty decent... although I haven't tried major functionality with spreadsheets yet.