The best thing until Halo 3 comes out!
That said on the campaign side of things you will want to play it a few times over,especially on co-op. The new cover system is easy and an important part of the gameplay. Epic manged to fit all these different cover maneuvers into one button which is nice for people who don't play shooters that often. The environments are very good and very much take on the role of fallen beauty. They are also quite varied and you will find yourself in five very different places during the course of the campaign. But the campaign mode was really made with co-op in mind and this is really where the game excels. All the flanking and other tactics that you cant do with your squad suddenly becomes an important part of survival. My one complaint about the Co-op is it makes it even easier as you can pick your team mate up if he dies.
The era of weapons is good. although there is not that many to choose from each is different from each other and they all have a cool si-fi twist to them.
Multi-player is fun but lacking in depth as there is only three very similar modes. There is lots of maps that are usually pretty small and a number of powerful weapon's placed in center positions or near both spawns. To finish off i just have to say that it is some of the best graphics that i have ever seen on any platform.....but who really cares about that when a game is this good