Pros:

Tower Defence in L4d
vmfs for the mappers 

Cons:

bad nav
ugly map

Conclusion:

Absolutly love it! Great job on adding new gameplay to l4d. 
Since the map design is very... functional ... i hope that this is some sort of test for further maps/campaigns :)
Keep up the good work! 

Oh, and thanks for releasing the vmfs aswell. Very cool!