I have. But I'm not sure what you mean by "declared a common-law partner"? You mean like just getting the document that legitimizes your relationship?
It's like very simple. There's this document (http://www.servicecanada.gc.ca/eform...8-01)e_cpp.pdf
) you provide the documentation for whichever option on (2) (3) or (4) you mark, you sign it and you get it notarized. Any notary public (and some immigration lawyers) can do that.
Takes like 5 min or less.