To me, the most useful aspect of Gdocs (other than the instant backup aspect, which is handy for brain dumps) is the commenting. ~ I agree that Word or OO is nice with the markup for that SP&G aspect of beta, but for the "does this plot make sense?" aspect (or if you're doing a "first read stream of consciousness liveblog" type critique) it's nice to have the discussion in the margins and not in-line.