As amazing as the first novel in this trilogy was, the rest of the novels don't hold up. Monsters of Men takes up right where The Ask and The Answer leave off, with the beginning of a war between the Spackle and the humans, who are themselves divided into two separate camps. In this novel, Ness decides to add a third narrative voice, that of The Return, or prisoner 1017, the only Spackle that survived the murders of the previous novel. While I enjoyed 1017's perspective, Ness also jumped around too much on occasion. The first 30 or 40 pages, not a single one of his narrators had more than two pages at a time, which made the novel a bit difficult to really get into.
I've also gotten tired of the relationship between Todd and Viola, and the fact that they would sacrifice the entire world for each other. While I understand that this is young adult fiction, it also just started seeming too unbelievable that these three teenagers (I'm counting The Return as one) would have such a say and an effect on the decisions the adults were making (I think that's one thing The Hunger Games did well - Katniss was a symbol of a movement, but when it came down to it, she was still a tool the adults used).
Also, Ness makes one other decision in the novels, and to me, it actually seems to weaken the rest of the series. It might just be me, but some of the things that happened in the previous novels were heartbreaking, and this decision actually reverses some of those things. It cheapened earlier losses. Also, this goes back to my earlier comment: the characters occasionally acted incredibly immature. I realize this is because they are about 14 years old, but given all the responsibilities the adults let them have, it is easy to forget how young they are. Due to this, when they make dumb decisions or harsh judgments, it is hard to be understanding. Instead of commiserating with the characters, I would often think, "stop being an idiot." As I said, this is due to the dichotomy of the characters' ages, and the way they are treated. If this had been the first novel in the series, I would not have recommended it. However, as the third in the series, everyone will of course read it just for the conclusion. It's not a horrible novel, but unfortunately, the trilogy as a whole does simply not do the first novel justice.