SPOILERS READ AT OWN RISK!
It was emotionally jarring (I was sobbing by the end and almost couldn't enjoy Percabeth's kiss!), and the twist was so unexpected. I loved how Luke was so human. Usually villains are very cardboard. They are told to us as evil, and that's how they act: evil. But Luke was different. Even before his ultimate sacrifice, he was constantly going back and forth between evil and good. He wasn't purely evil or purely good, and I loved how that complexity gave depth to his character.
*goes off into a corner to sob D:*