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At this point L4D2 has withstood the test of time. Genuinely excellent gameplay that more than a few zombie games afterward tried to follow suit or outright rip off to catch the same hype and player base. Even despite that, this game's quality as a zombie wave shooter goes pretty much unmatched. At the time of this review, long long long past this game's release way back 2009, you'll still see active players and servers with boat loads of modifications and custom maps to further extend its life.
All that aside, for anyone who actually hasn't seen or heard of this game, what is L4D? Its a 1-4 player cooperative shooter with optional PvP which doubles that player count. Get from your start point to the next safe house or extraction, clear objectives along the way. While most of campaigns keep their objectives the same, each time you start the spawns in terms of items and enemies are at the whim of the AI director which is more complicated an elaborate than you might think; even adjusting things slightly in your favor or against you depending on how hard a struggle your team has been going through. L4d2 also includes the campaign for L4D1 at this point which makes the previous game obsolete for the most part (unless you had some preference for the balance/exploits of the previous one). There is some overarching story through all the campaigns, but lets be honest here: that's not what anyone comes into this game prioritizing. The horde attacks can get wild and frenzied, really testing team cohesion and readiness especially with a variety of specials pitched into the mix.
In short: you can't go wrong with having this game in your library. Between the excellent design as a cooperative shooter, the optional competitive mode to play as special infected, and the staggering library of workshop mods now around to keep your game fresh even after you've cleared all the campaign maps to exhaustion I can say that this is Valve title that continues to outlive other titles that try to replace it.