I didn't care at all for this game at first, a friend of mine dragged me into it and then gifted it to me for XMas. What I experienced was a very fun co-op game but after a while I came to see its flaws which led to me stopping playing and deinstalling it.
There're basically two different game modes: co-op and versus (I count survival to co-op).
In co-op you play with others again A.I. controlled infected while in versus you compete against other players who control special infected.
The big problem is the lack of maps. There're only 5 maps which makes co-op soon very boring.
Versus doesn't get old fast but since the gameplay is heavily biased towards the survivors the fun doesn't last very long, either.
All this aside, this game became redundant with the release of L4D2 anyway, since the sequel includes every gameplay feature of this game and the L4D campaigns can be imported.
So yeah, if you're looking for a co-op zombie game buy L4D2.