I have only enjoyed the first title. I felt the others lacked inspiration and innovation and they too quickly fell into using the same formula over and over. I don't like respawn multiplayer so I never got into MP for this series.
As far as 4 goes, Ryan Payton recently left 343 because he wasn't able to expand and innovate on the franchise MSGS wanted more of the same. This is a huge red flag to me about the direction of the franchise. Halo 4 being the start of a new story should do nothing but innovate and try to expand its mechanics. The fact its not going to do this makes it not a game for me to follow.