Pros:

+ Not your average run-through-zombies campaign
+ Maps are challenging
+ Very different design-wise
+ Ending was awesome

Cons:

- Somewhat bland, though I'm sure that was the intent
- Platforming isn't really suited to L4D, especially when the game forces you to just hang off ledges, helpless
- Somewhat long a bit frustrating at times

Conclusion:

A fun and challenging campaign with a few problems, but no major issues. Oh, and do your best to avoid playing with bots.