Holt, Mira, Zoey and Max are the main characters. Three human (maybe--Zoey is under contention because reasons) and a dog. A dog. One of those loyal, intelligent, slobbering creatures who I can't ever remember playing a major role in any YA I've read. But in Midnight City, Max is as important to Holt and the other characters as an human, with a distinct personality that's all his own. I loved him and everyone else in this book.
Another aspect I loved were the Strange Lands artifacts. There was something scientific in the way they could be constructed to become different tool, something reminiscent of Chemistry and Physics that appealed to the science nerd that is me. And the author remembered that it took time to make these things, they couldn't be ready for use instantly, which added extra sense to the wonderful action scenes.
Any problems I had were small ones. The plot was slightly predictable sometimes--I could pick up a general idea of what was going to happen, but the details could still shock me. There were times when it felt like a particular phrase or adjective was used too many times in a matter of pages, which can get on my nerves but is also something I know I am guilty of in my own writing. And I need to know more about the aliens. Please. They're really interesting and I want to know their motives and how they work and everything!