With OfficeWriter, Reporting Services reports can be designed directly from Word and Excel with the OfficeWriter Designer, a special toolbar that is very easy to use.  This gives Microsoft Office users the ability to modify existing reports without requiring them to wait on developers, open Visual Studio, or even leave Office.  Office users can even create their own reports, but this functionality is disabled by default as most organizations prefer to use OfficeWriter Designer to help create a layer of separation between logic and presentation.  Under this paradigm, developers and IT professionals can create data models by joining data from various sources and placing it into a report definition where business users can modify the presentation of the report data to their heart's content (assuming they have been given necessary permissions).

Once designed, reports created or modified with the OfficeWriter Designer are modified with a special bit of base-64 encoded goodness that is "injected" in to the report's definition file.  On the reporting server, these extra instructions are completely ignored unless the report is requested for export via OfficeWriter's custom rendering extensions.  This means that any existing reports modified by the OfficeWriter Designer will continue to function just as they always did.  In fact, they can still be natively exported to a low-functionality version of Excel.  However, when requested for export via the custom OfficeWriter rendering extensions, the base-64 encoded goodness is used to recreate a native Excel or Word document that faithfully matches the template created in the OfficeWriter Designer.  With this template hydrated on the server, OfficeWriter renders extensions then replaces specialized "data-markers" with actual report data pulled from the datasets defined in the report's RDL file.  The result is a beautiful fusion of Reporting Services data into a completely native Word document or Excel spreadsheet that preserves native formatting and functionality as defined during design.

Even better, because OfficeWriter is just generating native Word and Excel documents on the server and programmatically populating them with Reporting Services data, it is possible to "intercept" rendered reports and use the object models exposed by OfficeWriter to programmatically add embellishments, modifications, and other formatting as desired.  In this way a single Reporting Services report can be customized for individual users or specific regions and needs.  For example, a sales company that wanted to give customized quotes to customers could use a single SSRS report to output data with various parameters, but contact information within the report could easily be switched programmatically for each sales associate creating the report.