There is no compelling reason to purchase it, if you are not a hardcore fan. It is not going to do anything for you the others failed to. The campaign and aftermath are not that long, as noted by others. The other modes are fun, but the number of maps is small. And the nickle and dime you strategy is the worst it has ever been.
The story being set shortly after E-Day, I enjoyed. And it was cool to follow up what Baird and Cole do while the others head for the island in the 3rd game. I would not mind a pendulum wars game either.
My son was meh about the game. But he likes overrun a lot. I think saturation levels have maxed out for him, like they have for many here. Just like Halo, at some point many that liked the franchise, have had enough of the style and story.
Speaking for myself, I like it well enough. The campaign is more frantic than any of the others. They throw more heavies at you, and far more often. The sawed off is better than the boom shot when it happens too. The magnum seems more powerful than it has ever been. The markza is a good balance of precision and ammo capacity. I rarely wanted the sniper because of it. The booshka is fairly lame. And being able to stick grenades by throwing them feels cheap. The new grenades are an OK wrinkle though.