I use OpenOffice and have done so quite actively for a couple of years, particularly for word processing and slide shows. But here I don't want to cover this very powerful application suite.
The OpenDocument format, in fact, is not simply the file format of OpenOffice 2. It is a document standard, formally OASIS Open Document Format for Office Applications. This is opening up the doors to many other office suites (for example, KOffice) to support the format.
In the past weeks, the OpenDocument format has been hitting the headlines mostly because of the decision by Massachusetts to chose the OpenDocument standard as their standard for office suite data exchange. Needless to say Microsoft is strongly questioning it, but with very weak arguments. There is a very intersting (and very long) article on the topic by David A. Wheeler on the Groklaw.net web site. Notice that the entire European Union is very close to adopt the same standard, probably after ISO ratifies it.
With the spreading of this format, only one player is pretending to ignore it, and that player is Microsoft. The reason should be obvious: they are making a ton of money with Office, mostly because they control the data formats used by most people on this planet. This is why an open and public format is strongly against their interest: they'll need to compete on features and price. But if things do change, they might end up adopting the open format. The OpenDocument Followship is running a petition to push Microsoft in this direction. If you like MS Office and want to use it natively with the OpenDocument format, you might as well sign it.