Pros:

Interesting world, full of well-placed details
Nice weapon placement
Mostly good path design
Nice dialogues (some parts could use some more, but that's personal nitpicking)

Cons:

Way forwards is confusing at times
IMPOSSIBLY unbalanced finale (WAY too many zombies attacking without a break, and it doesn' give you a breather even when the Tank comes)

Conclusion:

Play this until the finale and then abandon it. Seriously. I can only beat it on Easy, and almost always I'm the only one that escapes.

Pros:

Excellent use of character lines (only beaten by Blood Tracks)
Extreme attention to world detail
Overall balanced gameplay, considerably difficult without being broken
Multi-level map design (it's rare to see a not completely horizontal map, let alone a whole campaign)
Thrilling finale
Actual NEED of using the flashlight to find your way

Cons:

Two glitches:
1. The bots won't get on the train car at the end of map 4 (it's very common for one if not two to be left behind)
2. The radio conversation in the finale crashes the in-game dialogue (survivors won't say a thing even if they're being attacked by Special Infected) - not that big a deal in multiplayer, but a VERY big problem if you're playing with bots

Conclusion:

The bugs I mentioned, unfortunately, really cast a shadow in what would otherwise be a map that would take a perfect 10. Due to that, it's kinda problematic to play this campaign in single player, which I do the most.
I really hoped I could give this a perfect score, but alas, I won't if it doesn't deserve it.