Here's a wonderful explanation of why Microsoft's "open" OpenXML "standard" is a wolf in sheep's clothing:
In short, the specification is littered with little tidbits which say that to properly display a file you must understand details of how 12 year old software (Word 95) worked, or reverse engineer it. So here's a wonderful schematic on how to build a new house, but the kitchen won't fit unless you know how to arrange atoms at the quantum level, meagre saws won't work. And that, folks, is why I don't use Microsoft software and stopped buying their junk several years ago.
Update: Just to better explain why this is a problem, there is no guarantee that Microsoft is using its own specifications in its own software, namely Office 2007. In fact current reports state that they are doing their old tricks of using a mix of the current specifications with chunks of the older (undocumented) formats thrown in for good measure. Given this you'll have a slim chance of being able to use their published specifications and generate the same output that their own software creates. That is why people should use software that supports the internationally recognized OpenDocument format in either KDE's KOffice, Openoffice.org, StarOffice or the other programs that support it.