Due to the storage structure of the xml field, you might still have problems copy pasting content from other pages, no matter what editor you use.
As about how stable is the new editor, I suggest to give it a try to see how it works for your specific needs, and I can tell you that Liu and others are very active on this extension and so far have been quick to solve the problems that occured.
I am copying some content or tables from Microsoft word to online editor with ezpublish 3.6.1 and the xhtml code generated by ezpublish is embedding class="MsoNormal with paragraph tags.
Can anybody help, I am using 4.0.2 version of online editor.