Was Michelle Gomez brilliant? Yes, in that classic Who scenery-chewing mould. Highly entertaining.
Was Missy a good character? Yes. Crazy, ruthless, unpredictable, funny and charming - the hallmarks of a brilliant Doctor Who villain.
Did Missy ever feel like the Master? Right, I was adamant I'd say no to this, from the view of someone who's seen all the stories he's been in, and having grown up with him in the 80s. But the more I think about it the more logical it seems: he started as an urbane if megalomaniac kind of chap. Then he became a survivor, in all the worst ways you can survive. After that he just seemed to be trying to get the Doctor's attention - the Master's schemes were always far too elaborate to succeed. And then, by Survival, you got a real glimpse of his psychosis. And this psychosis, madness, whatever it was, was brought to the forefront in the new series, exacerbated by the things that happen to him, and it's escalated.
So I've talked myself round. Missy seemed too out-there to be the Master, but it makes total sense to me now.