I've heard bad things about this game. Heard it's no different from Halo 3, just a new campaign and a few new weapons. Not sure if I'm gonna snatch it up now..
Obviously it plays similar to Halo 3, because it's a halo title, but it has the VISR mode, which looks awesome. It has a different tone to it with the music composed, it has Firefight (shitload of fun), and it adds three new maps to Halo 3 MP, as well as access to the Halo: Reach beta (which is the next full Halo title, and is a prequel to the Halo trilogy).
So far it's been well worth the $60 to me. It has so much replay value with Firefight, as well as playin' the campaign co-op.