Left 4 Dead is a great zombie game. I definitely prefer this one over L4D2. Left 4 Dead has more believable characters, better atmosphere (plus it is easier to navigate), less of the “aw man, I have to be Jockey again”, more balance, and interesting locations. The downside is that the guns are not that great, but it is forgivable because it adds to the “panic” feeling.
Overall great game. I do advise that you wait until it goes on sale, so that you get a better deal.