Formulas, pivot tables, charts... essentially the menu or ribbon-driven stuff should be compatible.
VBA less so, because earlier versions of Office for Mac stayed with a VB5 code base long after Windows Office went to VB6 (Office 2000). And Office 2008 dumped VBA altogether; it was picked up again (and updated) when the latest Office version came out.
So, I guess it depends what you want to do with Excel.
Denis