It's good enough, but at times can drag on or feel too dense. If you're dying to read more about the Iron Warriors get it. Otherwise there are better stories.
Before reading this novel I remember seeing a review that mentioned how intricate McNeill's descriptions of the siege warfare and other battles are. I didn't think that sounded that crazy or different from other 40k books. I've read the first 6 Horus Heresy novels, and a sprinkling of other 40k books and decided to give this one a try as I heard it's one of the few that focus on the "bad guys" instead of the Imperium.
While it definitely does spend a considerable amount of time focusing on the Iron Warriors chaos space marines, it's more a split down the middle between them and the Imperial Guard. And wow... that is a lot of information on siege tactics and terminology. If you're not already familiar, you might want to brush up on terminology about trench warfare, siegecraft, and other military terms. It's definitely dense and sometimes that's to it's detriment. I occasionally felt confused about what McNeill was describing but just soldiered on because it frankly isn't SUPER important to the story. Bad guys shoot at good guys. Good guys die, but not all of them. Good guys shoot back. etc etc
I think the best parts of the story were when the story zoomed in and focused on a specific person (good or bad) like Hawke or Honsou. The only bad thing, is that most of the book is about grand, sweeping assaults on the main fortress instead of those individual conflicts.
Like mentioned at the top, read this book if you like the Iron Warriors, Imperial Guard, or even Titan war machines!