Besides e-mail, the only real office document I ever use is a spreadsheet. I’m a very happy Gnumeric user. My needs are reasonably simple; I share spreadsheets with Excel users and rely on some formulas.
It may not seem like an elaborate or flashy feature, but Gnumeric just works for what I need it to do. I’ve never had a problem reading or saving spreadsheets in formats that non-Gnumeric users can read. It works almost transparently, and the only reason I think about how often I use it is because I deliberately made a note of the applications I use the most.
There’s little better praise than “It works so well that I never think about it.” Thanks to all of the Gnumeric developers and contributors!