I loved this episode; I watched it twice in a row, almost without blinking. I love episodes where we, as the viewer, are able to see the Doctor through the eyes of someone who hasn't met him before, who doesn't know what he can do or what it's all about. It's almost like coming to the series anew.
Of course I could say all sorts of things about the awesome acting, fantastic special effects, etc... but that's just what I've come to expect from the show. This particular episode is great because of the fabulous writing and stunning suspense. I think it's one of the scariest, best episodes the show has ever had, and Doctor Who continues to raise the bar and get better and better.