I read it all in one day, yesterday. It was a great departure from the other Horus Heresy books because though it is about the Alpha Legion, it was a view of the Alpha Legion from the outside. From this outside perspective, the mysteries of the Alpha Legion remains. Even though a significant fluff/plot twist was introduced, it only increase their mysteries.
Overall, well written and better than any of the other Horus Heresy novels. I give it 8 of 10.