As a user of Open Office, I can attest that it's formatting of Word documents is far from perfect, and, there is no way to know how good a formatting job it is doing on any particular document. To get perfect rendering, I also use the free Office viewers that Microsoft provides for Word, Excel and PowerPoint. You can download them at microsoft.com/downloads (select "Office" in the left side column).
Despite the name "viewer" these programs also let you print Office files and copy data into other applications. The viewer programs are supported on Windows XP, Vista, 2000 and Server 2003.
The most popular viewers are those for PowerPoint 2007 and Word 2003. The Word 2003 Viewer, like Word itself, can read documents from earlier versions of Word.
The latest Word viewer, released in September 2007, is simply called the Word Viewer, with no version number at all. It's nice to be a monopoly. You can think of the latest Word Viewer as the Word 2007 Viewer since it lets you view the new .docx and .docm file formats. However, to get this functionality, Microsoft also requires that you install the Office Compatibility Pack.
The latest Excel Viewer, released in January 2008 also has no associated version number. A screen shot is below. I haven't used it much, but have noticed that it doesn't let you resize columns.
In part, this posting was prompted by a recent question at ask-leo.com - Do I need MS Office updates if I only have the viewers? The answer is yes, but Windows Update doesn't cut it. Just like with the real Office software, bug fixes to the viewers are detected and installed with Microsoft Update. The Word 2003 Viewer was released in August of 2005 and needs quite a few patches as shown below.
Office documents have often been booby-trapped with malicious software, so be sure to run Microsoft Update after installing any of the Viewer programs.
Many of the Word documents that I'm sent don't need to be edited, only viewed. If that's the case for you too, you may be able to save the cost of Microsoft Office by combining free Office software with the free viewers.