I have spent more than 500+ hours in this game so do I recommend it? "YES, hell, YES!"- BUT PLAY IT WITH FRIEND !!!!! The stuff i love about this game:
1) Well, first off it is completely free (everything in it).
2) It is unique and it tries to be as realistic as possible (unlike l4d for example)
3) It is unforgiving and yet very enjoyable (every mistake is punished but if you are doing well you feel like a boss!)
4) Brilliant animations (gun fire, reloadings, melee swings, healing animations, pill taking animations, suicide animations, ammo checking animations, )
5) Music (fits the game perfectly)
6) Maps (sewers, houses, ambandoned churches, large cities, narrow streets, grave yards, bridges and much more)
7) Constant updates and improvements!!!
+tons of other great things (kid zombies, realistic guns, weapon varieties, zombie traps in certain maps etc. etc. etc.)
-------------------------------------------------------Unfortunately, here are some cons:
1) Mentality of the players (sh!ttons of newbies, i mean thousands of them just wasting ammo and resources in wave1, then dying, and disconnecting- repeat)- I STRONGLY RECOMMEND grab a friend or two, show them how to play this game and enjoy it to the bits.
2) Hit detection can let you down...A LOT. (Yeah, the hit detection is terrible from time to time which means you can kill a zombie with one shot from colt1911 but sometimes you need more than 4 shots from the same gun. Luckily, after a little while you will learn how to deal with that)-> UPDATE: This will be solved in the upcoming update 1.9.4 !!! WOHOO
3) Infinite ammo servers (There is lots of them, they have destroyed the concept of the game completely, but on the other hand, there is still lots of vanilla servers with no modifications what-so-ever -> play there for maximum enjoyment)
4)Occasional crashes & tiny (1-2 secs) mysterious in-game freezes
So yeah...give it a try, you have nothing to loose ;-)