TL;DR -- I have trouble deciding if I should pass book along to my Dad as an introduction to the 40K milieu. On one hand the answer isn't a definite 'no', but on the other hand I shouldn't be having trouble deciding whether the book would make a sufficiently good impression for him to keep at it.
I can see why some people can't stand the book and yet others like it. It feels, and reads, like an abridged version of three middle books in a five part series, with non-existent first book helpfully summarized at the beginning to remind the reader what happened and occasionally alluded to along the way -- and yet at the same time the book dawdles along with considerably more contemplative navel-gazing than even I have a taste for! It doesn't help that I only agree with somewhere between one third and one half of the contemplative lecturing, and I'm trying to adjust for that in subjective taste, but the presentation of the dawdling itself seemed rushed (if that makes any sense) and I rarely got the feeling of characters chewing over ideas and coming to real grips and acceptance of them.
As (currently, August 2014) the earliest book chronologically in the Horus Heresy timeline (until when-if-ever the "missing first book" is ever released I guess??), it has a lot of potential for a great introduction to the whole Warhammer 40K milieu, or at least the Imperial side of it. And to some real extent it does meet that potential: it _almost_ counts as a shift between an ersatz version of regular Warhammer (medievalish fantasy knights on a crusade against nightmarish forces of chaos) into the vastly larger futuristic 40K setting. Its design also completely spoils what's going to happen, which I guess was a catch-22 for the author: almost anyone reading this book is already going to know where it's going, and I can understand a mandate to get there asap and not spend a (missing) book on the origin story of Lion El' Jonson since that would only distantly hint at the 40K elements to come, so why play around pretending things are going to be a surprise for the reader?
But on the other hand, this approach gutted me from sharing in much surprise and discovery by the characters. After all, most readers of ANY book will have some solid idea what's going to happen in it already from marketing promotions trying to convince the reader the book is worth reading; but that doesn't mean the author should, within the story, confirm and reveal most everything asap, and rapidly skip along the revelations by the characters.
So do I recommend it for the uninitiated or the initiated, either one?
Maybe. Meh-by. :/