While some of the new features in OS X Lion such as Autosave, Fullscreen, and Versions may be enticing to people who regularly run Office, for those who rely on some features of the suite you may want to consider the known compatibility issues between the various Office versions and Lion before upgrading your OS. As with any software package there are going to be some bugs that will need to be ironed out by the developers, and while most bugs are likely minor, a few of them may be enough cause a disruption in your work flow.
Over the weekend Microsoft published a knowledge base article that outlines some known issues with Office for Mac on OS X 10.7. Overall, if you have Office 2004 and rely on it, then do not upgrade to OS X Lion until you have an alternative Office version installed (2008 or preferably 2011). Office 2004 is PowerPC code, and as with Intuit's Quicken 2007 and earlier versions, if you install Lion then you will not be able to launch Office 2004.
Luckily there are options, including the ability to upgrade, or even install Snow Leopard in an alternative partition so you can still use Rosetta, but these may require you to either purchase new software licenses or set up a relatively cumbersome dual-boot situation.
In addition to the lack of support for Office 2004, there are a few situations in which Office 2008 and 2011 applications may crash. In Excel, a crash may occur when moving spreadsheets between workbooks. PowerPoint may also crash when you use Command-Tab in presentation viewer mode. The only other known crash situations involve Communicator (only in Office 2011), which may shut down when initiating calls or messages.
If you regularly use any of these options or application functions, then you might consider waiting to upgrade to Lion until they are fixed.
Also, some support functions might not work properly. The MSQuery tool for importing database information into an Excel document will not work in Office 2008, and will only work in English for Office 2011. In addition, the equation editor reference tools may not function properly, with visual problems or broken links. Lastly, Web browser viewing of files may not work properly, unless the browser you are using can open in 32-bit mode (to do this you will need to right-click the browser application in the Finder and choose 32-bit mode, if supported).
The other compatibility issues with Office include date formats not showing correctly, some interface elements not behaving properly, and labels for charts not being visible, though these may not happen at all times for all users.
The last bug is only relevant for people who have installed Office after upgrading to Lion: you may not be able to import messages from Apple Mail into either Outlook (in Office 2011) or Entourage (in Office 2008).
Despite these problems, Microsoft has been tackling major compatibility issues with Office for Mac since early in Lion's development, and currently the Office programs that run in Lion should do so quite well. I have been running Office 2011 and am able to perform all of the functions I regularly use in Word and Excel. However, if you choose to upgrade to Lion, be sure you are aware of these known issues and keep an eye out for Microsoft updates in the near future to address them.