It's intensely powerful (my boss can do spectacular things with pivot tables and teaches all the business people to do this as a means to doing their own analysis of our reports) but the high-power features have a positively nasty UI compared to the corresponding SQL query.
I'd love to see an Excel designed-from-scratch that scratches the same itches of approachability and user-friendliness but with an awareness of Excel's failings and without obsessing over the spreadsheet metaphor. Something in-between Excel and Access.