Depends on who you ask. It's not the Defining Masterpiece of Western Civilization that Microsoft's hype campaign promised us, but we're still talking about a very fun game with lots of customization and replay value here.
I really do wish they had put more effort into the campaign, though. I remember playing Halo 2 in co-op for the fourth time or so, thinking how much better the sequel was going to be, but turns out they put virtually all of their effort into the multiplayer mode. ;___;