Left 4 Dead is already an legendary game, very good, but what L4D doesn't have is Steam Workshop, which L4D2 has. With workshop the game is on another level, you can make a whole another game with workshop, it's worth buying it, especially when there is a sale going on... You can play online with other people, with friends, anyone you want, there is versus, survival, pretty fun, when you mix all that with custom campaigns and mods you have everything you need...
Would i recommend it? Yes! Every. Single. Time.

Left 4 Dead is already an legendary game, very good, but what L4D doesn't have is Steam Workshop, which L4D2 has. With workshop the game is on another level, you can make a whole another game with workshop, it's worth buying it, especially when there is a sale going on... You can play online with other people, with friends, anyone you want, there is versus, survival, pretty fun, when you mix all that with custom campaigns and mods you have everything you need... Would i recommend it? Yes! Every. Single. Time.