A lot of people have complained about the Doctor and Martha being offscreen for most of the episode. For my money, this is what makes the episode so strong, because the focus is on a character who doesn't have any experience dealing with some of Who's more alien and bizarre monsters. Most of the credit goes to the genius of Steven Moffat's script, which creates what is an incredibly scary monster, one so easy to escape and yet so difficult to defeat.
But a lot of the success rests on the shoulders of Carey Mulligan, who basically carries the story as Sally Sparrow. Good premise or not, without such a likable actress in the role, it wouldn't have worked nearly as well.
So this is a story that is best told on a dark and stormy night, with those you love close by your side.
But whatever you do, DON'T BLINK!