That is one of my favorite episodes I've seen so far. It is such a great introduction to the characters. I love how mysterious and even hostile the Doctor could be then. It gave him an edge that I liked. Good old Billy Hartnell.
I also loved Hartnell's little speech about being wanderers in the fourth dimension. Hartnell truly gives weight and mystery to it. I wish the BBC had saved those lost episodes. I also enjoyed Carole Anne Ford's performance as Susan. She starts off as interesting, only to end up as the screamer. That was a shame to me.