My son (age 6 and a half) is a mad keen Doctor Who fan, and loves this book. We have read it together several times to get different outcomes by making different choices and decisions during the story.
The author has captured the Doctor and Martha's personalities very well, and you can just imagine David Tenant and Freema Agyeman saying the things they do in this book.
A bit of mystery, and a not TOO scary plot makes this ideal for the younger Doctor Who fans. And you get good value with the ability to re-read and change the story each time.