Pros:

 + Glitch free 
 + L4D1 Common Infected in L4D2
 + Makes l4d2 campaigns have a L4d1 atmosphere
 + Fully compatible in all maps
 + Online support

Cons:

Absolutely nothing!

Conclusion:

I applaud Tails1 for his effort in creating the perfect L4D1 experience in L4D2. I also fully support Tails1 for his job with original work.