Played with the web version for a bit. Nice looking app, but doesn't really do anything I need that I can't do with Google Docs, which my company uses for a lot of collaborative writing. We still eventually publish the docs out to Word and PDF because that's how they get consumed, but for the collaborative writing part it's Google Docs all the way, so I see value in refining the collaborative writing market.

Having offline mobile is "nice" but not totally necessary for my use case, mostly because the number of people I know who car write decent sentences on their mobile devices are few.