Somewhat poor AI and it gets boring sometimes, but still the best zombie shooter out there.
Any game built on Source is going to have a little trouble catching up with modern, advanced graphics, but even though it's a couple years old, L4D2 is still comparable to modern games. Rather than using higher res textures and more complex models like most people do for more advanced graphics, L4D2 uses strategic lighting and atmospheric effects to overcome the inherent drawbacks of the Source engine. It may not be Crysis 2, but it still brings you a great atmosphere that conveys the apocalypse and, of course, gallons of beautiful blood.
The game mechanics are fairly simple to learn. You can carry one primary, in addition to a pistol or melee weapon, and multiple accessories. Each of the weapons has equal drawbacks and advantages, meaning you'll never be duped on munitions. It won't take you long to learn that this is not a shoot-em-up game so much as a run-the-hell-away, as players who attempt to clear areas will soon find themselves dropping like rocks.
There isn't really an involving, dynamic plot behind this game, but considering the type of game, Valve did a great job at juicing out whatever of a story they could. In addition, each of the characters is likable, from Ellis, who may remind you a bit of Mater from Cars, to Coach, whose humorous personality makes him a great companion for the zombie apocalypse.
But where this game really shines is in its multiplayer. As if simply playing through the missions with friends isn't fun enough, there are plenty of gametypes to mess around with. For example, you may be scavenging gas cans as an enemy team attempts to tear your brains out, or you may be taking turns as survivors and zombies in the campaign missions.
On that note, the zombie mechanics are just as easy to learn as the survivors', although a bit harder to master. Still, pounding your helpless foes into the ground as your teammate strangles the person trying to save them always gives you this warm, fuzzy feeling inside.
The game has some drawbacks, but definitely not enough to ignore the low 20 dollar price tag. For example, the AI can be a bit retarded - I remember being pounded to death by a charger by the top of a manhole while my retarded AI teammates kept trying - and failing - to climb the ladder through it and save me. There's also the fact that the AIs never use tossable items such as grenades and molatovs, which makes them much less useful than (at least, capable) human teammates.
In addition, I always have a hard time playing the game for long stretches of time. After maybe a game of versus, which takes roughly 40 minutes, I quickly lose interest. It may be a personal thing, but the fact that most of the campaigns just involve running and gunning quickly becomes dull.
All-in-all, it isn't anything worth dying for, and I wouldn't pay the 50 or so dollar price it had when it first came out. But, it is now cheap enough to give you no reason not to give it a go. A great game, and hands down the best zombie game on the market today.