This thing is much more back in the areas where I feel more comfortable and already use the tools widely.
My current work uses Google Docs for a number of student projects: they're fantastic for collaborative science labs, or for recent projects like our History Day plays. Students can work on different parts of the document (or even the same part) easily, without having to track versions of the file.
I've also been looking at solutions for a personal project where I'm writing a bunch of group documentation. I tried Zoho Writer for this: I like it a lot (though I'm seeing some weird spacing issues/formatting issues when I take it into PDF. However, that's manageable, and since my ideal would be able to keep an editable version somewhere web-accessible, and send PDFs out as needed, that's okay.
I found Zoho faster and more responsibe than I've found Google Docs, but I know this is both sometimes a factor of specific technology combinations, and of changes in the software, Google Docs has been better when I've tried it recently. I do wish both sites had a more accurate method of doing very basic layout tasks (keeping sections on the same page, for example) other than printing it to PDF and seeing what happens.