An overall average story. The Seven and Ace interaction was fun, and the story did end up having some interesting things to say about the nature of the Doctor. However, the story was just way too long, and too overwrought for my tastes.
Perhaps, if the fear creature had been inside the Doctor all along (or inside Ace), the story could have been turned into something intriguing. As it is, "The Fearmonger" ends up telling us things we already know about human nature.
I love a good, edge-of-the-seat paranoia story, but the execution of this one was rather ho-hum. The Fearmonger isn't bad, it's just boring.