L4D2 is a must have game for PC gamers. It is sure to provide you with endless hours of fun, has good replay value and has 4-player co-op as well as pvp multiplayer versus mode. There are also thousands of player created maps, campaigns and other mods you can download and play - http://www.l4dmaps.com/
All L4D1 campaigns are included in L4D2 so no real point of buying the first game unless you really want it although there's a 2 game bundle on the US storefront for $7.50 if you have a friend in the US or are able to purchase through a proxy.
"L4D2 is a must have game for PC gamers."
That's a pretty big call. Lol I look forward to the others, fanbois(?), chipping you on that. :D