I've been using it to write production code for almost 6 months now. Over the last month, they've eliminated all the minor annoyances. It's solid and works. I couldn't complain even if I wanted to. The codebase is also very approachable. I needed a feature, wrote it, pull request, and they accepted it.
With a traditional IDE, you have to install it everywhere. With me, that would be a half-dozen computers, with 500MB lost on each one. With Cloud9, it's installed on the server and I can access it anywhere there's a net connection. Updates are centralized, you only need to do it once.
And it's funny, but the IDE is actually more performant in the browser. Well, I was using Aptana before, which is essentially Eclipse and that's known to be slow.
Also, it just feels natural to have your IDE right next to the end product. IDE in one tab, web app in the other.
In fact, Ace (the Ajax.org Cloud9 Editor) is what Github uses for their file editing interface, so if you're editing files on the site directly, you're already using Cloud9, partially.
I had to use Cloud9 a few months ago to populate a new repo for a small project I wanted to share. With today's changes I wouldn't have needed to use Cloud9. Being able to add files is long overdue and I wonder how much it was to stem the tide of users using third party sites to populate github and thereby becoming entrenched in a development/publishing workflow that only used github as a storage layer and not somewhere where you would spend time or send others to the github website.
