Depends on the school, your major, and the individual professors. In my classes (physics), I do not accept Word documents-- PDF's only. Also when I work with research assistants and most colleagues we use PDF, plain text (often LaTeX) or basic Word documents (i.e. the kind that Pages has no issues opening).
I think the "share-ability" of Word documents is greatly overstated. I used to have quite a bit of trouble going between different computers with Office on them. Font issues, margin issues, etc. If all you care about is the text, it doesn't matter what package you use. If you are worried about formatting, distribute PDF's. When I do collaborative writing, we always separate the content from the formatting, which is done at the end by the first author. So it doesn't really matter how material gets passed around-- PDF's, plain text, latex, simple word docs.
Basically, try them out and find which one you like to work with. If you like iWork better than Office, get both. The academic version of office is cheaper than some textbooks (man, textbook prices really piss me off), and maybe your school has Office for even cheaper (my university offers for $21).
crackpip