I am not that impressed by the game play. There is nothing new in terms of game play. It's quake for console. Single player campaign feels like quake 4's campaign.
I like co-op though, I just wish it was handled like GOW co-op. I understand why halo 3's co-op is as it is, they have the option to score so someone joining int he middle screws up the score. But then have it so when in scoring modes that no one can join, but when there is no scoring some can join in the middle. Set it up so that person can't get the achievements, you can only get achievements if you started the level.
The game is all about multiplayer. It's still run and gun like quake or UT, but that isn't what makes it special. Halo has tons and tons of options. I am also enjoying the forge. A real map editor would be better, but that is a decent substitute for now.
I also play with a good group of people. I would have traded in Halo 3 by now if it wasn't for
2old2play. If I had to play only against random kids it wouldn't be fun.