I mostly use the exact setup you're talking about. I can't really speak to the "teamwork" aspects; for that I generally use CollabEdit, which is simpler for one-off collaboration. I got a Chromebook because I was planning on being in fairly impoverished areas in Central America for months or years, and I wanted a laptop that I was not going to worry about breaking. It works pretty well, all things considered. It's relatively simple to install a 'real' linux distro via crouton and get access to all the normal linux goodies. One specific advantage to ChromeOS is that it keeps track of what apps you have installed, and if you ever have to replace the unit, you can just sit at the new one, type your login info, and in about two minutes the new machine will have exactly the same stuff the old one did.
Having your development tools/files in the cloud means that they are inaccessible to you without an Internet connection, however, you don't need much of a net connection to be able to work: for Cloud9 there's an initial download of I believe about 1 MB for the editor, and actually editing code is possible down to a hundred bytes per second. Creating a local repo from a GitHub or Bitbucket repository is very simple, and each coding workspace gets its own little virtual machine, so you can install gems, run tests, and do anything you'd normally do. It also saves process state, so you can start (e.g.) pry, fool around with the interpreter, close the window, and the next time you start, pry will still be running. It actually saves quite a few brain cycles: you have less effort to figure out what you were doing the last time. Code completion and refactoring support exists, but is not what you would call world-class, more like SublimeEdit than intellij.
I have been using cloud9, but I have shopped around for various online editors at times, and so far I have not found any particularly compelling reasons to switch. I do not miss setting up a new chroot or container for a new project, or worrying about syncing code between workstations. Also note that there are online IDEs which can be run on your own private server (Cloud9 among them), for a hybrid approach, and of course there's nothing wrong with emacs over ssh if it comes down to it. At this point I doubt I would go back to a "real" IDE unless required to by an employer.