This book stands out as being the only one in the Diablo series of books to actually feature characters from the games, and to have any more than a tangential link to those plots. The Sin War series is set 3000 years before the games, and the various stories in the Archive are either non-canon or barely linked to the central storyline as portrayed in the three games. As such, regardless of its other qualities or faults, you actually feel like you're reading a Diablo story rather than a general fantasy story with "Diablo" written on the cover.
I'd agree with the otherwise ludicrous 1-star review in that it is pretty slow going at first. I feel the author doesn't really convey exactly what Deckard is trying to achieve or the urgency of that task, and as such you're spending your time waiting for a hook that stubbornly refuses to appear for a few chapters. Any other complaints are, in my opinion, typical of all such licensed novels and aren't really particular to this book specifically. The writing style can feel quite pedestrian, some of the descriptions fall a little flat and sound silly, giving the impression that the author is trying too hard. For example, his description of Hell, as viewed by one character, is meant to sound maddeningly depraved and horrific, yet rather than the Lovecraftian terror he reaches for, you just get a kind of mild disgust - more akin to looking at a bowl of worms that the darkest recesses of Hell. Again, those are things I've found in all licensed, franchise books like this. Of the Diablo books, this is the least guilty of those faults, Knaak's being the most guilty by far.
Cain's world-weariness is conveyed well, you get the feeling that he's just a dusty bag of old bones, animated only by the importance of his task. Leah comes across as haunted, distant and lost. She's been abandoned by her biological mother, then brought up to the age of 8 by a woman slowly being consumed by the awful memories of what happened in Tristram in the first Diablo game; a woman who seemingly alternates between hating Leah and being terrified of her. The fondness she later speaks of "Uncle Deckard" with is developed in this book as she goes from pariah to surrogate daughter of creaky old Cain. It's interesting to see the contrast with her warmer and more open personality in Diablo III, and frames her reluctance to accept Cain's belief in the prophecy of The End Times as less a lack of trust and more an attempt to block out her traumatic childhood memories.
This book is solid rather than spectacular, and while not exactly a fantasy classic, judged as what it is - further backstory and lore in the Diablo setting - it's a great success and has been rated accordingly.