We are finding that PowerPivot is an excellent tool for performing this 'discovery phase'. Not only does it unearth the data sources its also gives you a heads-up on data quality and potential master data management issues which tend to throw a spanner in many BI projects.

As a BA it allows you to get a far better understanding of the business process and how the data is used to support it, and it allows you to literally sit and design reports with the business users and get their buy in - user adoption is key to maximizing ROI. It is often said that business users don't really know what they want and that with BI projects requirements change very rapidly post-deployment once users actually get their hands on the data and the tools to analyse it. PowerPivot allow us to deliver useful results and built a good rapport with the stakeholders very quickly.