Pros:

+ Very good map layout and design.
+ Length of maps was fine. 
+ Plenty of challenging moments, lots of zombies.
+ Good weapon variety, well placed weapons and ammo for the most part.
+ Interesting finale with the colours changing and the Alien place. Reminds me of something out of Crysis. 

Cons:

- There is a problem with the Bots using the ladder in the final map, having played through the campaign twice, the same thing happened, they would fall down, or get stuck at the top and another bot would climb back up, then fall down, this wasted medi's for the finale. 
- Lack of Ammo at the finale, if there is some ammo, I don't know where it was. 

Conclusion:

Overall very good, one of the few good futuristic/alien related campaigns I have played. It's fun, challenging, and is about the right length for a campaign (took about 1 hour 10 min on Normal). Everything was well laid out, easy to follow directions and fun to play. The only draw back for me being minor glitches with the bots on the ladder in the final map, which was quite annoying in an otherwise glitch free campaign.