I just got to play this game after holding off for a year waiting for co-op.
It definitely exceeded my expectation with how fun it is (just the campaign, I'm more of a SP/co-op kinda player).
The sandbox is quite good, with all the weapons, vehicles, and gadgets, especially the zipline thing.
It's probably my favorite Halo game and also the best FPS game I've ever played.
Too bad about the launch though, seems they were trying to do too much and also hamstrung themselves with the old gen consoles.