Pros:

-Really long campaign, up to more than an hour of calm gameplay.
-Very good Half Life 2 concept.
-Mixes very well with Left 4 Dead.

Cons:

-At the underground parking, there are too many infected that spawn at the panic event, thus making it very difficult for some players, not for me, but for some others.
-Too many Tanks spawn at the finale. One is enough.

Conclusion:

This is one of the best campaigns I have ever played. I love it and I'm sure everyone else does love it as well! If only there were a sequel.