I don't use Office if I can help it. Being Microsoft is one thing.. being totally clunky and ridiculously bloated is the other. Word and Powerpoint both piss me off. For school uses, I write in Pages and do my presentations in Keynote (Apple's iWork). Can open and save .doc and .ppt. Thankfully I have no use of Excel either. Other than that, for normal writing purposes I use a simple text editor.
OpenOffice is great but ever since I started using a Mac, it was too ugly compared to what I am now used to. Heheh.