I gave Crash Course a run through just now, and I think it's pretty good. Sure, it's just two chapters, but dang, they're long, intense chapters. They feel almost two chapters long themselves.
I played it on normal, and while I had little trouble mowing down the waves of normal mode zombies, I did take quite a number of hits, so I can see this campaign being quite difficult on the harder difficulties.
Also, [spoil]The finale has an interesting concept to combat turtling. The generator shuts down again mid fight, forcing you to fight your way back to it, and power it on a second time.
![Razz :P](./images/smilies/icon_razz2.gif)
[/spoil]