I thought it was one of the easiest of McNeils books to get through. I've covered his flaws many times.
I really liked how the Emperors children were portrayed. As for warming to their characters. I really didnt want to and I think the idea was to have a fascinated revulsion with them. Lucius is ALL about his showmanship - its all about taking their perversions to an extreme to gain perfection. That's one legion full of really sick puppies.
I think more could have been done with the Raven Guard and Iron Hands but it was only a sub plot. I thought it was done well enough that these guys deserve to show up in future books. I certainly want to hear more.
Eidolon was a weak character in the book. He didnt really feature, but again I thought fabius was great. Maybe it just comes down to expectations. I have very few as I've never really been in to the lore of chaos warriors and such and before the HH books came out I couldn't have even named the traitor legions let alone their characters.