Was there anything wrong with it? No, not really, it was basically the same as all the existing campaigns, you run through a city in an eternal dusk setting shooting zombies and trying to find you're way to safety. I have only two minor problems with the game:
1. The campaign was way too short, consisting of two maps, one set in an industrial/warehouse district, prompting you to run through a maze of storeroom, offices, and alleys due to street blockage caused by jackknifed semis and the sort, and a finale episode that entails you to run through a fairly extensive shipping yard to get to a vehicle designed to travel the streets safely. Seriously though, if you're escape vehicle is a ten minute run from the safe house, can this really be called a safe house? Seems to me this is just a house, but I digress.
2. Guns and ammo are everywhere to be found. On the surface this may not seem like a problem, but the degree to which weapons and ammunition are scattered throughout the levels was one of the main atmosphere setters in the original campaigns. You were always aware of how many rounds you had left because that's how close to death you were at all times, but Crash Course apparently had an M-16 fairy run through the entire map sprinkling her certain brand of love. It got to the point that I was more worried about hurting myself tripping over piles of ammo in the street than the zombies getting to me. The one thing they did do is limit the health packs, however that's still not the same thing as limiting the firepower; as long as I had one point of health left I could continue shooting, you never had to worry about being defenseless AND hurt.
Along with the guns and ammo, there were an almost unlimited supply of gas cans and propane tanks laying around to shoot, not to mention the pipebombs and Molotovs; I think this is a sign of Valve trying to cater to the instant gratification crowd, giving more and better destructible environments, something I personally don;t need, but it's no biggie. One feature I really liked though was in the finale, you're required to flip a switch to start a generator, bringing a horde of zombies down on your team; the catch was that you have to stand there in the open for about 10 seconds doing nothing button hold the switch in order to activate the generator. On top of that, you need to do this again in the middle of the fight in order to restart the generator. A lot easier said than done.
So there you go, all in all it's a good add-on. A quick campaign that can be played relatively painlessly. I would also say that the maps seem to be longer than any of the original campaigns and I think the graphics are a little smoother. My opinion is that this is essentially a beta for Left 4 Dead 2, what you see here is what you'll be seeing there, based on the screenshots I've seen of the new game.
Since it's free, as long as you have the PC version, there's no reason not to get it and play it. If you have the 360 version I would seriously consider waiting for L4D 2 and saving the 500 odd points that Microsoft is charging.
