I have ONLY played this in co-op and I only have 20 hours in it.

I always thought that Left4Dead just HAD to be overrated in some way, because how could a worse version of the Call of Duty horde mode be good- but that's the thing, this isn't that- this is better than I could've ever imagined. Somehow this game is good. I don't really get it, but here we are.