Three major points of criticism:
1. Bechdel Test Fail
Up to about two-thirds, there weren't ANY women in this book. After that there were three: One scolding her husband for putting their son at risk, one serving her husband a meal and telling him everything would be okay when he was in a bad mood, and a third one offering the protagonist her little son in exchange for ammunition (iirc). Oh, and not to forget, there was the diary of a dead woman that was read by the (male) protagonist.
As you can see, women really played an important role in this story. Miserably failed.
2. Extreme overuse of Dei Ex Machina
Whenever the protagonist found himself in danger, suddenly a total stranger (or a group of them) appeared out of nowhere to help him: When he wanted to leave a station, when he wanted to enter another station without having the proper papers, when a station was attacked, when he got lost in the tunnels, when he was captured... Every. Freaking. Time.
3. Falling short of expectations
You would think that the setting of this book would make for a pretty creepy read - Moscow's metro system after an apocalypse, its stations inhabited by only a few surviving humans, surrounded by mutants on the surface that are always about to attack, also frightening beings in the tunnels and malevolent groups supervising certain stations - but it doesn't. There was solely one scene in the whole book that I found a bit scary, that's all.
Oh, and the resolution wasn't ingenious either.
Two stars for the unique setting and because it wasn't completely boring.