Try code like this:
CKEDITOR.instances.editor1.document.getBody().getText();
It works fine for me. You can test it on http://ckeditor.com/demo. It's not ideal (text in table cells is joined together without spaces), but may be enough for your needs.
EDIT (20 Dec 2017): The CKEditor 4 demo was moved to https://ckeditor.com/ckeditor-4/ and uses different editor names, so the new code to execute is:
CKEDITOR.instances.ckdemo.document.getBody().getText();
It's also important that it will work in the "Article editor" and in the "Inline editor" you need to get text of a different element:
CKEDITOR.instances.editor1.editable().getText();
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