Left 4 Dead is the game you play with friends because surviving the zombie apocalypse is sadistic fun.
The gameplay is fast and furious. I'm a veteran FPS gamer, but shooting thousands of zombies never gets old. That, coupled with the AI "Director", ensures that what you play tonight will not be the same playthrough tomorrow. There can be times where you might get a passing sense of deja vu ("hasn't this happened before?"), but those times are few and far between.
Because the game is crafted around co-op, ignore the single player. You should not buy this game for a single player experience. It would be redundant. Not to say that it is bad, but you miss out on the entire L4D experience.
The graphics, for the aging Source engine, are beautiful. The music, while somewhat repetitive, is controlled by the Director as well so it comes and goes as appropriate with the action. The controls are what you expect from any PC FPS.
This is a game you play because you enjoy screaming over the mic because a wave of zombies just busted through the vent in the wall and you wake up your family in the process. It's the game you play because you like the occasional challenge that a nub on your team will provide you because you and your friend happen to be elite gamers. And finally, it's the game you play just to have fun. You can relax, and just shoot stuff. Stick together, use your head, and act as you would if you were in the characters' shoes, and you'll have a blast.
This is where it's at right now folks -- it's time to hop on the zombie express.