Pros:

The 4th map
the 5th map
music queues

Cons:

A ballon at a ship port when there are high up buildings seems odd
movement on the ship is awkward
prison didn't feel like a prison

Conclusion:

Before I say anything else I will try to tell you how good the 4th map is .... I love that map so much I am worried I might get it pregnant.
The 5th level design is genius, the only things I could find wrong with it were a missing path bug (if you stand on the boxes on the pier tanks would target you but were unable to get up) and that it was a little odd that a ballon would get to a ship pier when there are high up buildings where he could dock.
The first 3 levels are good too but they simply were nothing special. You have alternate routes, a few panic events and a prison without cells. I have to stress this, they are not bad they simply are in the shadow of the 4th and 5th map.
My conclusion: Download it, if they had voice acting you would assume they came from Valve.