I rented this game not knowing anything about it or the Metro (book or game) series. It turned out to be very interesting and a solid, and satisfying game. Short completion time (6 hours casually) with 2nd replay value, as there are different challenges and a secret ending.
The stealth play against humans was very fun (think thriller, first person Splinter Cell, with no climbing), but fizzled out in the last third of the game as you switch to primarily fighting overly aggressive mutants. Where humans can be knocked out, or head-shot killed, the mutants will test your battle technique, patience, and of course, eat all of your ammo.
The weapons and upgrades are okay, but nothing to get excited about. (Grab a decent shotgun, Sniper, and automatic assault rifle for best results).
Storyline starts off very strong but ends leaving much desired in character and plot development.
Sloppy AI, and annoying surface and mutant levels. I found that I had to motivate myself to play-through the surface levels, so that I could get back to sneaking around and killing humans.
All-in-all, a good game, worth a Saturday on the couch, or Friday or Saturday night at home while the kiddies are sleeping.