gnumeric is not bad, not at all. As I write this, it still suffers from sever printing problems, probably due to Postscript generation. Today, to print a fairly small worksheet gnumeric allocated 64 Megs. I had faith and waited for the end of the swap fest, and it made it, but it was not pretty. mpg123 greatly suffered :-)
User interface compatibility with Excel is outstanding, down to the keyboard equivalents (or accelerators). I have seen a Windows Excel user (non-geek) pound out a whole tax worksheet on gnumeric without one instant of hesitation.

Obvious as it may be, gnumeric is a GNOME application that runs under Linux.