The Excel add-in is one of two just released Business Intelligence solutions available for download:
These are free of charge to licensed customers and include full documentation and PSS support.
The aim for the Excel Add-in for Analysis Services is to provide a fuller reporting solution for OLAP cubes than is natively enabled in Excel PivotTables. You can use it to build a report structure with different grouping and segmentation options, and you can then fill that structure with data from an OLAP cube.
Some nice features:
If you're looking for a spreadsheet-based client into Analysis Services and you're finding PivotTables slightly too structured, this add-in is well worth evaluating.