Just tried C9 again -- You can change text in open files, but you can't open new files (doesn't seem to cache everything) and I couldn't refresh the browser view of my web app. To be fair, my app pulls some assets from CDNs, but if C9 gave me cached versions or just failed to load them I could have kept working. I'd say at this point, fully offline is too limited... at least if your use case involves many files and building web apps.