Because this is a game tie-in, you have to expect the story to be just a little bit weak and that the author would have to pad it out somewhat. That having been said, Roberson deviated from the game plot, added extra characters and somehow still managed to make it far more boring and uninteresting than a simple retelling of the game would have been. He even missed out important characters such as Derosa and Gabriel Angelos!
Story and character changes aside, the writing style is simplistic and dull, and for a book that's mostly action it's incredibly tedious. If you enjoyed the style of C S Goto's previous trilogy, you absolutely will be disappointed by this one. There's also a distinct lack of Eldar, odd considering the game plot pretty much revolved around them.
Overall: mediocre, badly written and entirely uninspiring. I would strongly recommend picking another WH40K author such as Kyme, Dembski-Bowden or Abnett and giving them your money instead.