Is left 4 dead worth it

The answer is yes. L4D is a fun game which will be upadated to make better. It has a wide variety of players from many countries to play with.
You can either play with bots in in campain mode against your PC.
You can play online with people in a campain, I would suggest using advanced difficulty for the best gameplay. An lastly you can play versus mode where you can play as the survivors, or the infected where you can play as the special infected, eg... Hunter, Smoker Boomer. In versus mode you can also play as the boss infected, the tank.
Basicaly, the aim of a survivor is to advance to the next saferoom, and finally get rescued, and of an infected is the stop the survivors by killing them.
The variety of weapons is a bit bleak, (Shotgun, SMG, Pistol, Uzzi, Huning Riffel, Auto Shotgun, Pipe bomb, molotov.
Pipes are useful for fighting off the hordes, as the zombies are attracted to sound, and wil just run to it and get blown up. You should allways save a molotov for the tank, so you can burn him, which makes killing him so much easier.
There is so much more to the game, and i really would give it a try.