HALO 3 is definetly a system seller. Gears of War was great but HALO 3 makes you enter the world as Master Chief =)
When you have finished the campaign at several difficulties, you still have lots of fun playing it online in various modes. You can even customize the online maps!
If you are not a first person-shooter hater and love the look of the Marines from the movie ALIENS, this is definetly a must have!