Office is barely compatible in my experience, across 2003, 2010, and 2011.
Even documents that look very simple, like a Word document with a picture, would open perfectly in 2010; open with a blank space where the picture should be in 2011; and not open at all in 2003 even with the compatibility pack installed.
Excel files work fine, so long as they don't use macros. Stuff created in 2003 doesn't work at all in 2011, but it does in 2010; stuff made in 2010 sometimes works in 2011, but not 2003; stuff made in 2011 doesn't work anywhere else.
After these problems, I didn't even try Powerpoint. If my work makes me use that, I'll use it exclusively at work, and watch telly when I'm at home.