I had the opportunity to use Google docs, in a truly collaborative fashion. I'll say its better than many things I've tried.
It auto-saves, which is good. Some of those saves trigger revisions, had to get used to the notion that a revision isn't necessarily anyone's conscious choice of a publishable delta. Top that with wiki-style diffs, whitespace and cross-browser quirks, it can be hard to follow changes.
Some revisions just state "no text added". Fun.