Great game with some pretty obvious flaws.
Good stuff:
1.) Lots of hidden stuff, weapons and such.
2.) Different ways to progress.
3.) Many types of nemies.
4.) Hard.
Not so perfect:
1.) Narrative is scarse and uninteresting. Too little story-points and information. (The could be some dialogue or notes, diary-pages etc to collect...)
2.) Too many enemies with insane amount of healt. Battle goes on-and-on.
3.) Micromanagement. If the earlier levels are fortified, where are the hordes coming from? It's boring to go back to the easy stages just to clean them up regularly. Why is there no-one else to shoot down the zombies with a minigun? Why should the hero bother? And yes, I have bought the recruites and updates, but the factories build up the tension.