Editorially is great and I almost used it to collaborate on a project; however, it's missing two big features (which I'm sure they're working on):
1) Shared folders - each draft has to be manually shared with people today.
2) Non-owners can invite - currently ONLY the person that creates the draft (the owner) can invite other people to view/edit the doc.
I'm still a die-hard Draft (http://draftin.com) fan.
I love their culture (their blog is fantastic). You can feel this culture in the tool - simple and clear. However I agree with @rrhoover - some features are missing to make it powerful.
I can never decide between Editorially and Draft. I love them both so much. I think Draft has gotten me farther in the writing process but Editorially is the better-designed product.