While this book benefits from incorporating scenes that aren't normally in the film, there are several weak points that detract from the overall enjoyment factor of this novelization.
First, the strengths of the book. Showcasing the origins of Cyclops and Storm (which are radically unlike the comic book versions), adds a fun element to the beginning of the book. Also many of the movies scenes were expanded in this format, which creates a fun `directors cut' feel for the fans that have seen the movie several times. Another strength is since so much has been changed from the comic, anyone can enjoy this book as a stand alone from the comics, or the movie quite easily.
Now, the weaknesses. It feels as if Rusch and Smith, who normally are wonderful authors, suffered by receiving an early script that hadn't been revised. Many of the key scenes that made the movie so wonderful are changed, and not for the better. The result is that the characterizations suffer. The characterization of Logan suffers the most. The dialogue is wooden, and often, the writers take points that could have been done subtlety in one or two lines and beat the readers with them like a mallet. The worst mistake is when the according to the book, Wolverine extends the four Adamantium claws of his left hand... a wretched mistake for a casual fan to make, let alone an X-Men author.
Overall, the movie is far better, and if you're looking for a fun X-Men read, there is a host of X-Men trade paper backs and graphic novels that would provide a much better reading experience.