This debate has been covered extensively before. However, if you are still unconvinced that Microsoft's Open XML (OOXML) is nothing but a proprietary format in disguise, have a look at the lastest from Groklaw. This blog item, which equates this 'standard' to a "monopoly enabler", has even gotten Bob Sutor's attention.

...I learned this week that despite Microsoft's promises of a new openness and its assertions regarding interoperability for its OOXML (formerly known as OpenXML and also known as EOOXML) and despite having offered it as a standard, it seems that it's another case of promises, promises. From what I've been reading, which I'll share with you, I think it's time to ask ourselves some serious questions: does OOXML really qualify as a standard? Or is it yet another monopoly-enabler in the guise of a standard?

Sadly, Novell has given Microsoft's plan some credence by saying it would offer OOXML support.