One of the great tragities of the Horus Heresy was the treatment the Thousand Sons Legion and its primarch, Magnus the Red. Magnus who was one of the most loyal of the Emperor's sons, and one of the few that had a direct and personal link to the Emperor (through their shared psychic powers -- Magnus had been in communication with the Emperor before he was found), but Magnus' delving into the ways of the Warp set him up for his downfall.
McNeil does a very good job in presenting both Magnus as well as his Thousand Sons Legion in a very sympathetic light. Where most of the Primarchs we have seen so far have a personality flaw that in one way or another makes them unlikable, Magnus' flaw is his faith in his knowledge and his hubris that he can master this knowledge and power. This makes Magnus seem very confident (a likable trait usually), but he is also compassionate, reasonable, and committed to preserving knowledge and culture. Similarly the Legion and many of its personalities (such as Ahriman, which long time players of the Warhammer 40K wargame should be very familiar with) share this desire, and for me I found myself often agreeing with their actions.
Like a Greek tragety, however, Hubris deserves a fall, and Magnus indeed falls. Contacting the Emperor to warn him of Horus' trechery via the forbidden art of Sorcery (the Council of Nikaea, presented in the book as a virtual trial of Magnus), the Emperor sends the Space Wolves, supported by the Sisters of Silence (null-psykers) and a detachment of Custodes, to destroy the Thousand Sons (which makes one wonder about the two "lost" legions, if perhaps they had a similar fate...).
One element that I had to question was the subplot involving the psychicly aware Remembrancers. Their story was interoven with that of the main plot, but in the end we do not discover what their fate was. I understand Dan Abnett, author of the upcoming Prospero Burns had fallen ill, and was unable to complete the book by the deadline. Originally this book was to be released in April, but has been pushed back till much later in the year. It is possible that Abnett would pick up on this plot point, but due to unforeseen circumstances we are left hanging.
Overall this was a solid book in my opinion, and can't feel a little disappointed that the second part is so delayed.