Vinay Patankar wrote a detailed and comprehensive review of three software for collaborative writing.
He compares Quip, Google Docs and Microsoft Word with regard to major features of writing in collaboration such as sharing, concurrent editing, revision history, chatting and commenting, and organization.
He rates Quip as the best among the three software. Based on my experience of using Quip for the last two years, I think he is right.
Check out the review here.