Pros:

-Lets you kick anybody from the lobby as long as your lobby leader.
-Makes the menu look a lot better.
-Adds new mutations to the game.
-You can change your add-ons while in the game.

Cons:

-Only people with the mod can play the custom mutations with you. (-.1)

Conclusion:

It makes the game a whole lot better and adds WAY more features than the regular Left 4 Dead 2.