Pros:

Well thought out and put together maps
Limited access to powerful guns makes first 3 levels challenging and exciting
Limited access to ammo meant you needed to use pistol often and only switch to main gun in big battles.
Beautiful maps with nice textures.
Original and different story concept.

Cons:

The story concept of going down into a dig site in a military base doesnt make much sense to me.  If I am running away from zombies why trap myself in a lair underground.
Final battle was less stressful than some of the earlier levels.

Conclusion:

I liked it and had a lot of fun playing through it.  The first 3 levels shutting the door to the safe room was a definitely *phew* we made it moment.  Without paying the original voice actors its a bit difficult to convey the reasons for being there.  So I am happy to go with willing suspension of disbelief.  The final scene could have been improved with some upper level galleries dropping zombies opposite the main entrance and allowing specials to attack from higher up. The maps all looked fantastic and had atmospheric lighting.  Definitely worth the download.