(1) Ease of Use: While computer users throughout the world (including this author) have become familiar with Microsoft's Office suites over the years, OpenOffice is not difficult to learn by simply using it. It's long been kept a secret, but no training in basic use of Office suites is needed; only advanced use of an Office suite may create a need for training -- regardless of which suite it is.
(2) Tailored Solutions: OpenOffice has the benefit of permitting more customized applications to interact with it due to ithe Freedom associated with the source code, which means it will allow more people to develop custom applications which interact with it. Microsoft products require more Microsoft products to interact with them, they come at a cost and limit what a developer can do since the source code is not available.
(3) Better and Faster Work: Such comparisons are inherently flawed, since they would have to have the same users doing the same work on different Office suites. Let's face it: Users just want to do what they have to with their software. In this regard, OpenOffice facilitates this just as Microsoft Office does, but has the benefit of having the source code available for allowing more applications to interact with it. This means more potential productivity when dealing with the business logic of a SMB.
(4) Seamless Data Exchange: Microsoft claims seamless data exchange within Microsoft Office - but it's only between people using Microsoft products. OpenOffice allows people who use a variety of operating systems and data formats to interact with each other. Microsoft Office does not.
(5) Easier Deployment and Maintenance: Installation for either package is very simple. OpenOffice does have a clear benefit here: Service packs are not something one has to constantly look for (at this time). Further, simply installing the latest version of OpenOffice over a later version takes less overall time than constantly updating via patches and service packs.
(6) Security: Microsoft is brave to bring viruses into its marketing strategy when it has been one of Microsoft's greatest problems, despite all the nice things their Marketing brochures have to say about how secure it is. Where the rubber meets the road, Microsoft Office loses.
(7) Investment You Can Trust: Using OpenOffice is an investment of your time, your energy and your future of being able to interoperate with people around the world, without worrying about what operating system that they use. Microsoft Office is an investment in Microsoft's time, energy and future.