Maybe 3.5 stars since I'm in a good mood.
It was a very interesting idea, but I found myself having to put the book down to have a break from my building boredom with the story. Something about the novel's premise just did not captivate me as it should have.
I am disappointed that the complex characters of the Ninth Doctor and Rose were not handled as well as I would have hoped, with no character development at all. While I understand that they are fully "developed" characters already, it is still true that everyone learns something from their adventures.
I think that if it had not been for the Doctor Who element, I would not have continued reading past the 40% mark, but I'm glad I did as it was wrapped up nicely in the end.
Overall, I would say that the main thing missing from this is a love for the subject matter; without that passion for Doctor Who from the author, the characters fall flat and the story just never gets off the ground at all.