Although I have never played the game, X-Com, I found this book to be very fun and exciting to read, and particularly enjoyed the geographic descriptions of the locales. It felt as though the writers had either spent some time there, or else done their research very well.
It was not all rosy, of course; there could have been a bit more description of some of the aliens and some of the starships; some of it felt as though it relied upon the reader's prior experience with the game. But all in all, the book was exciting, action-packed, and a real page-turner, and kept me guessing right up to the end as to who the traitor really was. I definitely recommend it.