Left 4 Dead does a lot of things right that make it worth checking out. It's aesthetically pleasing, works the concept of the "post-apocalyptic zombie flick" to maximum effect, plays great, and offers lots of fun for multiple players, online or off. As a first-person shooter, it's enjoyable, as a multiplayer game, it's good times, and as a zombie-based horror-themed game, it does everything it needs to do to get itself over well.
However, it's also limited in size and scope; between a lack of character variety, a lack of playable levels, a lack of enemies, and a lack of weapons, the only reason to keep coming back IS the multiplayer, as you'll see everything the game has to offer in a few hours. If you're a fan of playing online with friends or online FPS titles in general, Left 4 Dead is fabulous, but if you're looking for substance over style, it might be worth renting first to see if it has enough to keep you coming back.