The fact that Children of Earth is still rattling around my skull days after I watched it, the fact that certain scenes replay over and over in my head because I'm so impressed with them (no, not That One), and the fact that the story makes me think about the real world as much as it does the Whoniverse is, for me, strong proof of the series' quality.
Here's another quite brilliant take, by the talented selenak . (Many thanks to honorh for pointing me to the essay and the essayist.) Go. Read. Respond.