Back to School is a full featured Left 4 Dead and Left 4 Dead 2 campaign. The storyline is not interfere with none of this games and offer completly new setting of Borden city, struggling to survive in an epidemic apocalypse alongside with original 4 heroes. They are getting a chance to save themselves, since USA marines are coming in town and not falling back this time. But during the battle apogee something goes terribly wrong and local school - the most safe place in city - turns out to be a death trap.
This review does not contain Spoilers.
This campaign was absolutely outstanding. It's first big impression was the fine attention to detail. The graphics were fantastic and incredibly well optimized. Even in areas of high volumes of particles I still never dropped below 30 FPS on my mid-range machine.
This was one of the (if not THE) most creative, original, and overall memorable campaigns I have ever played in the entire L4D series. I wouldn't even hesitate to say that it is better than a good handful of Valve's own campaigns. There was a lot of variation in the environment so it never felt as if it was dragging on. There was never too much repetition, there was always something new and exciting after every turn.
While it is obvious that some of the puzzle elements were very experimental, I think it was executed pretty well, despite the game constantly having to point you in the right direction (which is pretty typical of L4D anyway). To top it all off, the end fight was very creative, entertaining, and unique. The campaign was much longer than I anticipated though, and took me almost 2 hours to finish.
I am a very critical judge when it comes to map work in Source, and even manage to find flaws in some of Valve's own works. I could not find anything - ANYTHING - flawed that is even worth mentioning in this campaign. The biggest flaw was probably a single bright cubemap in a dark area or something, but Source always does that.
That being said, 10/10. 3 years well spent, good job guys.