Pros:

love having tower defense in L4D

Cons:

In base defense:

-fences and things don't always place correctly
-sentries don't seem to fire at things

Conclusion:

Very creative, 10 for innovation alone.

Pros:

awesome!  adds so much more functionality to L4D!

Cons:

the other people have to have it too, but that's expected.

Conclusion:

If you only get one mod, it should be this one.  btw, is there a list anywhere of what all the mods do?  also, what other mods do you recommend using with this one?