Well, Excel, ... Gnumeric is said to be more compatible to Excel than Excel itself.
But,
not all macros pass, not all graphs pass, ...
I personally find Gnumeric a better spreadsheet than OOcalc. Same non-compatibility btw.
There is an Open Document Architecture, the FLOSS version is comprehensive in OO, unfortunately Redmond designed an Open Document Architecture which is said better ('course you pay a license fee!) hence no full compatibility.
Now, as these are compressed XML files, you can always decompress them and mull them through sed.