L4D2 . . . I bought it but haven't put more than ten hours into it since the release date. It's fun but not anything different than the original L4D. There's basically nothing new in there other than a tweaked game engine that couldn't be added via DLC in the original.
And now with more news on the way about pay-for DLC content coming to L4D2 . . . no thanks. I understand the business model but I don't support it.