This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
Our FAQ, AccessingFileFormats has your options as far as interfacing with Excel are concerned. I usually generate a CSV file since it is much easier to work with compared to XLS. Unfortunately, it doesn't support formatting or formulas.
Once you create the file, you can send it to the client with StreamResult.