If you're on the open source side you'll probably be using Open Office instead of Microsoft Office. But did you know there is more than one version of Open Office?
Go-OO (that is, Go-Open Office) is a fork of Open Office, that is, a special version or development build that features some interesting improvements regarding the suite it's based on.Among these improvements you can find support for OpenXML and Microsoft Works files, an optimized Microsoft binary file support, and perfect import of Wordperfect graphics, among others.
You can also work with Excel VBA macros and Visio diagrams.
Other than that, Go-OO is visually identical to the original Open Office suite and includes exactly the same apps, namely Writer, Calc, Math, Impress, Draw and Base. If you need the specific improvements included in this special version of Open Office, give it a try; if not, you may as well download the official version because there's not that much difference.