Chris Claremont gives this story what the movie could not: depth, details, and a true love for the characters. Small wonder, given that he created and developed many of them to begin with. The novel allows for a view of the insides of many of these characters' heads, most notably that of Jean Grey/Dark Phoenix. The exploration of her psyche throughout the conflict is fascinating. Rogue and Warren are also greatly improved in the book: Rogue regains her spunky, sassy self, and loses the pathetic weakness she's taken on in the movies, while Warren is made more active, more useful, and given more than three lines.
I adored all the little things Claremont included here -- just a few words that he would toss off, referring to a single incident from comics that were published years ago. The inside jokes made it feel more like my beloved fandom again, not a Hollywood travesty.
For all of that, though, the book loses none of the action or tension that were the strong points of the movie. If anything, it clips along at a better pace, in the spirit of a good adventure read.
I recommend this book especially to those fans who were disappointed by the movie's handling of their favourite characters, and to long-time X-Men fans who will appreciate all the small referneces.