Having finished the entire trilogy, I'm left feeling a little lukewarm about it. This series came highly recommended to me and everyone that I know who has read it loved it. I definitely liked it, but I'm not sure that I'm willing to go as far as "love."
For me, part of the problem was the style of narration. When reading The Knife of Never Letting Go, it took me a while to get used to Todd's stream of consciousness narration. In The Ask and the Answer, Ness kept the same stream of consciousness style but alternated chapters between Todd and Viola. I initially really liked this, because it gave me some insight into Viola, who I thought was a great character. In Monsters of Men, Ness adds another layer, with stream of consciousness chapters switching between Todd, Viola, and the Spackle known as "The Burden." This was a little much for me. In some places, the chapters were very short, and I felt like I never truly got into any of the characters' voices before needing to change gears.
The title of the book comes from the premise (repeated several times throughout the trilogy) that "War makes monsters of men and women." The Mayor's oppressive regime and growing attrocities in the previous books led to war in this book. And it was definitely war on a grand scale--with four separate fours to contend with (the Mayor's army, the Answer's terrorist forces, the Spackle, and the ever-looming threat of a spaceship full of settlers), all of whom changed alliances several times during the book.
In spite of all the action, though, I was frustrated by the middle of the book. I felt like it was a lot of "same old, same old." The characters continued to have slight variations on the same debates that they'd had for the previous two books, and eventually things reached such an impasse that I felt like there was simply no way for the book to resolve. While these traits may well be true of conflict and war in real life, they didn't make for great reading.
While the last hundred pages did include some developments I definitely wasn't expecting, I wasn't really satisfied with the ending. I actually feel this way about a lot of dystopian books--they've painted a society so bleak that the best they can do is end with a slight glimmer of hope, but definitely not a feel-good sort of tie-up.
I would still recommend this trilogy to anybody who likes dystopian novels, but with the caveat that it wasn't my favorite trilogy/series out there.