We tested it internally, then on our beta site with some user input, and finally it went live.
You must understand that web technology, especially browsers, are all a bunch of interpretations and hacks built upon old buggy code and more hacks. Browsers do crazy weird things on some people's machines, while on others they work just fine.
From what we can tell the new editor is working for about 95%+ of the user base we have. That isn't bad, considering the old browser probably worked for 90%+ and had its own little quirks.
The reason we ask people on these forums for their input isn't so the editor can be tested for the first time. It is so we can address the issues that come up for a minority of users. Issues that we can't reproduce internally and that need specific, sometimes very creative, workarounds.