So I surprised to see this in the marketplace last night, seeing that Left4Dead 2 comes out in November. But being the zombie killer I am I had to download it. It does add another stage of single player but it's only half as long as the other ones. The old chapters might take an hour where this on only took me 20 minutes to beat. Luckily it does have more achievements with it so that should at lest get you to play through it a couple times.
My Score: 7.0