Just started using Paper, and I really like it so far, especially for documents with high degree of collaboration / cross-referencing like meeting notes and project plans (person and document mentions, hashtags, date tags, etc). Editing experience is also solid in Firefox, which is more than I can say for Google Docs. Markdown auto-styling is a nice feature for me as a coding guy, but that's a matter of preference.
My biggest gripe with Docs, though, is that documents are impossible to find (ha!). Sharing does not seem to preserve folder structure, and I can never get relevant search results for some reason. I am yet to discover if this is implemented better by Dropbox.