I've done a few co-authored pieces. My first big one, i.e I was the one who wrote it all and did all the physical research, I am currently working on. My sup keeps changing mind on whether I should write for publication or thesis so I tend to start, then stop then start thesis writing, then stop then start publication again etc. etc.
I think my writing style has definately improved by resubmitting and working on other papers though so guess I will wait and see