I just finished my first game of the new DLC (The Passing) and my first thought is AWESOME.
Why you ask me? Very simple I just got 20% more game for free! and if thats wasn't enough the new campaigns is one of the better ones from the six campaigns if not the best. It seems Valve took a good look at the best parts from Left 4 Dead and made them into a whole new level for instance the storm and rain effects from Hard Rain make a comeback. The levels in the DLC are very diverse and fun, and one last treat is that you get to see and meet some familiar faces.  
In conclusion I just want to say play this DLC even if you have to pay for it (im talking to you Xbox playes) its a ride thats worth every second of your time.    
Sorry im keeping this short but these zombies aren't going kill themselves.