Saw the first couple "Sensorites." Susan is acting much more mature, and neither Susan nor Barbara are screaming at every shadow they see, which is rather refreshing. As soon as it's discovered that the Sensorites use fear to take control of humanoids, I immediately thought, "Oh, crap, Barbara and Susan are toast." Fortunately, they're being written much more competently in this outing, at least so far. I like the part where Susan cuts off Ian who's trying to explain a spectrograph. She already knows.
I actually feel like these episodes sort of help explain her childish behavior. I get the feeling that the Doctor has been sheltering her for a long time. It makes me curious about how she came to be in his care, and how he came to have a family in the first place. I'm fairly certain the original series, at least, never addresses this, so that's a bit frustrating. I remember that the prevailing feeling back in the 80s was that Susan couldn't possibly be the Doctor's blood relative, that she was just another companion, one who had grown close to him and began calling him grandfather. Yeah, right. Hartnell and Ford were playing it as if they were indeed grandfather and granddaughter, and so I accept that that is the case.
I like the dynamic now of the four characters all on the same page after starting from a place where Barbara and Ian were in constant conflict with the Doctor, with Susan stuck in the middle of it. They've been through a lot together and they're a true team now. This is character development we don't really get with the later Docs. They get their companions, who last a year or two, then they move on to the next one. And though Tom Baker is still my favorite Doc, Hartnell's Doc in less than one season has had more character development than Baker did during his entire run.