Just finished reading Graham MacNeill's novel of this name, and holy freaking cow was it good. While it doesn't really advance the main storyline of the Horus Heresy that much (Indeed, from a couple of throwaway lines, it appears this is set before Battle For the Abyss). I did like the way it tackled the Death of Innocence in a way that was logical and helped explain how come the Loyalists didn't simply destroy the traitors thanks to superior munitions and supply chains.
I also liked the way this used the Emperor/Omnissiah dichtomy to fuel the friction between dissident factions within the Mechanicum itself.
Now I have a desire to collect a Legio Tempestus Titan battle group in Epic: Armageddon...