The 6th novel in the horus heresy series detracts away from the style of the previous 5 novels, and takes us back to the beginning of the dark angels legion. As a result, around half of the novel reads like a warhammer fantasy story, with the 40k setting kicking in later. for this reason I think, many people didn't like it. Like the later book Battle for the Abyss, I was put off by the reviews and actually moved straight to legion, even though I brought Eisenstein and Fulgrim along with this, but I am glad I went back to this and made up my own mind.
Overall it was a very good book, and one if its main strong points was that, unlike the other novels, the story almost exclusively focuses on the one character, or at least from his point of view, rather than jumping from character to character like the others did. I agree that the ending was quite abrupt, and I sensed that the author ran out of space, and had to 'sum up' at the end to keep within the desired page count, but if you know the fate of the dark angels from the codexes or elsewhere, there are few questions left to be answered.
I'm not sure if it will be made into a trilogy as some rumours say, but even if it isn't, enought has been said here for the rest, as the saying goes, to be history.
Overall, an entertaining story, but if you expect an exclusive sci-fi novel, you might be dissapointed by the earlier parts. On the other hand, if you enjoy a good story, and don't worry so much where the setting is, you won't likely be dissapointed.