I'm currently attempting to write a roguelike using nothing other than Excel formulae (no macros etc.). It's going surprisingly well so far, although the limitations are obviously frustrating - not least the inability to correctly lock-in random numbers, although I think that can be worked around by linking the data from another workbook.

Obviously this is not a serious project, but it's kind of fun. I think this is where Excel gets dangerous. To a certain type of mind (perhaps like mine), the enjoyment from forcing a non-compliant system to do your bidding is greater than the enjoyment of using the proper tool to do the job quickly and effectively.

I still don't use pivot tables. Those are too practical.