I feel like this and my opinion(s) may differ from yours but this is what I think after playing it just now. It's an ok campaign. I loved playing with the old L4D characters but it was a bit short. Then again, I guess you really don't need like 4-5 maps to tell the story that left off in the L4D2 "The Passing" DLC. I feel it's best played on L4D2 more than L4D because you get the new Special Infected. Some of the achievements are something you more likely have to plan out to get or have your friends help in a private VS. game. I haven't played the "TAAANNNKK!" mutation but from what I saw from some of the broadcasters I watch on Justin.tv... It's a real bitch. Unless you're a pub then you may have a chance to survive to the safe room. One drawback I wish Valve did was allow you to play the L4D2 characters in the No Mercy campaign but I guess the community made map will have to do for me. Again, I'm not saying it's a bad DLC. It's ok for me.
Feel free to post your thoughts on it, PC/360 users.
