One of my team members forwarded me this article on building VSTO-Based PowerPivot workbooks. The article is based on the work we did for our PowerPivot App for Twitter in Excel and it is written very well. Worth the read if you are interested in implementations.
“Microsoft Visual Studio Tools for Office (VSTO) is an awesome development suite for building advanced Office solutions, such as Excel applications that go above and beyond default features and capabilities. Even our PowerPivot for Excel add-in is based on VSTO. However, it is important to note that VSTO does not define application programming interfaces (APIs) for applying customizations on top of customizations. In other words, you can develop applications on top of Excel by using VSTO but you cannot develop applications on top of PowerPivot because VSTO does not include any APIs for this purpose and the PowerPivot add-in does not add any such APIs either. Appropriate APIs are only available with PowerPivot for SharePoint and SQL Server Analysis Services.”