And since you actually mention Excel, why not use it?
Linky of all the things i'm about to mention :
http://en.wikipedia.org/wiki/Office_Web_Components
Look into things like "Visual Studio Tools for Office" , where you code .NET Panels inside Excel. The Excel API is fully accessable via .NET anyhow, so you can code it right into your app too. And Pivot Tables in Excel are actually VERY easy to program, alot easier than populating a excel sheet the normal way [insert data in cells/range etc etc].
Also, i'm not sure if Microsoft relaunched it [i read it somewhere]. but look for "Office Web Components" . Microsoft actually released the Excel Pivot Table components as controls which you just embed in your web app...it's basically the Pivot Table+Graph without the Worksheet.
Obviously all these means you will be required to code the actual "how the data gets in there in the first place" yourself. So you'll probably need to code a little reporting class...especially if you want to make it dynamic [i.e. users create stored procs that populates a dataset which are loaded into the control/worksheet]
Reporting Services on the other hand have a little ad hoc report builder where users can design their own reports on the fly [directly from the tables or views [or whatever you provide] ] and save it back as static reports. However this is not really a pivot table, more like "I want a table on here, with these columns, sum that column, add a graph, make the heading this, filter like that"...basically the entire report....but it's pretty user friendly though.
The Office Web Components have been discontinued in Office 2007, and are not included, except as a part of Office Project Server 2007.[1] However, they will still be available for download from Microsoft's website. Microsoft has not yet offered a complete replacement for the Office Web Components. However, programmers can use a combination of third party products, Excel Services or Visual Studio Tools for Office to provide similar functionality.
