Pros:

Original and creative concept
Length of the campaign was just right
Interesting stuff to see (boomer hanging from the ceiling lol)
Challenging

Cons:

Easy to get lost
The finale...sigh

Conclusion:

Okay, it may have taken me a while to complete it and finally post a review on it, coz I for one never managed to find the time to play through the whole campaign at one shot, and also coz...well, I had a tendency of failing halfway (the disadvantage of playing with only three bots to help you lol).
Especially at the new Witch area, it took me many many tries before I finally managed to get past that part.
But at least it proves that this campaign is challenging, and I love a good challenge.

After playing through so many other custom campaigns, this campaign was like a breath of fresh air really, and it really was a welcome change from the conventional campaign, where you just kill zombies along the way to reaching the next saferoom, to the next one, on and on before finally escaping...bleah.
The unique-ness of this campaign really makes it stand out from the others, and that is definitely one of its major plus points.

There were a lot of mini-events too (I believe you call them easter eggs?) that happened during the game that added a nice touch to the campaign and really helped stamp down this campaign's reputation as a truly unique and outstanding one.
I shan't go into detail on this part so as not to spoil the surprise, but do look out for them! (like the one mentioned above, haha)

However, sometimes everything just gets real messy and you have no idea where to go. There isn't like any fixed direction set for you to follow and you end up running around in circles. Especially in map 1, after you enter the compound it's just like a huge mess of metal and debris. Add that to the dark environment, and you're in for a frustrating time.

Anddddd the finale. I personally felt it was a letdown really, after just going through what was actually a pretty decent set of first four maps.
I have mixed feelings on the sentry guns, coz although I applaud such a unique idea in L4D2, I felt they were so good at what they do (killing zombies) that it really just took an edge out of the finale, like it wasn't as challenging as it was made out to be in the end.
The lack of Tanks too in the finale left me kinda disappointed, though I do love how that one Tank made its appearance in the finale (totally wasn't expecting that, haha)
Also, I know that that is one huge ass helicopter that's going to bring us to safety, but still...so many gas cans needed?
Okay, maybe the number wouldn't be such a problem, IF they weren't so spread out literally all over the place. I think the finale area is waaaaaay too big, and transporting the gas cans to the chopper gets super annoying after a while with the constant long trips back and forth. Basically, I think everything would actually be fine if the size of the finale area was reduced. Correct me if I'm wrong, but I think the distance you would cover during this finale transporting fuel cans as compared to that of Dead Center is definitely much longer. And at least in Dead Center you are able to drop the fuel cans down to your escape vehicle to avoid the inconvenience of having to push your way through say a whole horde of zombies with a damn gas can.
If the finale can be improved, this campaign would definitely be one of my faves.

8.7/10. I would really really love to give it more, so please give me the chance to do so, coz right now I really just can't find the reason to.