I love this game, it's extremely addictive and has something for everyone. From single player, for those achievement lovers, to Versus for playing as infected against human survivors. These are only 2 of the game play modes, the others are really good including a "Mutation" mode, which varies and offers different unique game modes.
In comparison to L4D the original... It is better in some regards, for example the new levels, weapons (Including melee) and of course the new special infected, but if you played the original game you will probably agree it's still great for vintage reasons =)
A definite recommendation for any zombie lovers.