I confess to not being a fan of OO - leastways the Mac version. Clunky, often cryptic, and inclined to invoke the beachball of death at any moment. That said, it's free and the utilitarianism is probably not going to hinder the occasional letter writer.
I like Office, mostly. It's polished and I've never had formatting issues moving between versions (or platforms). I have no issues with paying for software and if there's one good thing that OO has done it's reduced the price for normal mortals to something close to reasonable. That said, Office 2008 for the Mac is a whole load sucky (Excel without pivot tables and VBA, you may as well take it out the back and shoot it). The version of Office 2007 that the mothership beamed onto my work PC is fine. I even quite like the 'ribbons', the old UI was getting labyrinthine.
That said, for non-work use I like iWork. Pages is a breeze to use (my god, graphics stay where you put them), Numbers is a innovative and intuitive approach to spreadsheets (though it lacks anything like Excel's power to crunch through huge piles of unsavoury data), and Keynote laughs in the face of Powerpoint.