Dick Kusleika pointed a few days ago to a paper by Charley Kyd. In it, Charley gives a convincing overview of the possibilities revealed by OLAP-powered Excel solutions. Speaking from experience, I can only attest to the amazing things this combination allows you to achieve.
I’m surprised by Charley’s statement that TM1 and PowerOLAP “return data to Excel about 100 times faster than Analysis Services does”, but then I haven’t tested either to compare it to SSAS coupled with XLCubed. This combination is so fast already that I’m not sure how a user would pick up a 100-times increase in query speed. The article is nonetheless very much worth reading, particularly if the combination of OLAP and Excel sounds new to you.
If you think you’re familiar with cubes because you’ve used Business Objects before, you may want to check out my post comparing Business Objects Universes (and cubes) to the cubes provided by Microsoft Analysis Services.