The following comments are owned by whoever posted them. I liked it a lot. There are many aspects to Bonobo support in Gnumeric. This will become the default build shortly after 1.4 is released. Embedding gnumeric as a Control or Embeddable will be improved and full support within the new workbook model-view framework should go in when I get back from vacation.
Any serious uses require graphing.
It is included in most distributions, I think, and allows you to fit any function to data, not just a limited set. I came from Excel myself, but now I always use Gnuplot. The only problem is that it isn't integrated in the modern GUI framework - you have to export the data in a simple text file and process it with the small Gnuplot script from there. But there is a rather nice Python interface to it that makes it quite a bit pleasant. Or, if you like matlab-type interfaces, Octave also uses Gnuplot quite nicely.