I liked it!
I liked, that if You let one of zombies get inside, You would hear Your currently equipped weapon being fired a few times.
I loved the chopper part, upgrades made sense.
My favourite weapon was of course lmg, I only used smg when I was completly out of ammo or had a clinger and wouldn't reload lmg on time. Combat shotgun was crappy.
By the way guys are tough - mutants can run faster than a car, jump higher than a gazelle on steroids, and when they grab one of soldiers, not only they don't manage to instantly break their arms, but get punched in the face and die! I think a knife stab or a pistol shot from the driver in a quick time event would suit better.
Also it would be nice if shooting of both of zombie's arms would also get it of Your car - that way disposing of helmet using zombies would be easier and more fun.
Art was really great, I also liked that You could rescue some civilians, instead of egoistically leaving them to their doom in first part. Even if You know, that the city is going to be nuked.