I just downloaded the 24 hour trial from Steam and played a few games. The zombies can easily out number you. They come from every angle, and when I say every, I mean, they will reach up and grab, reach down and grab you, throw stuff at you, come up from behind you.
I like zombie games. The difference in this game is that the zombies are much smarter, faster, more powerful than I've seen before. If you try to go in alone, you won't survive very long.
Here's the Zero Punctuation Review, if you haven't already seen it:
http://www.escapistmagazine.com/videos/view/zero-punctuation/459-Left-4-Dead