I recently spent some time checking out the Visual Studio Tools for Office product, a.k.a. VSTO. It allows Visual Studio developers to add features and capabilities to MS Word, MS Excel, and with the release of Visual Studio 2005, also MS Outlook.
The advantage is that the code to add features or capabilities to these environments can be written fully in Visual Studio using standard C# or VB.Net code with full IntelliSense assistance and all the other advantages of the .Net framework. No need to learn VBA (Visual Basic for Applications) or macros to be productive.
The disadvantage is that VSTO only works with Office 2003 or later. This makes it unsuitable for widespread commercial applications at this time. A small disadvantage is that your assembly must accompany the document as it is moved around the network or emailed in order to maintain the functionality.
Conclusion: VSTO and the Visual Studio integration in SQL Server 2005 are key steps toward the programmer's nirvana of "learn one tool well and use it everywhere". Its impact will be even more positive as the majority of home and business users upgrade to Office 2003 or later.