TGDaily says Halo 3 is definitely the Xbox 360's killer app:

It is quite undoubtedly the best single-player campaign of the entire Halo series, and anyone who buys the game needs to run through it at least once before getting too involved with the multiplayer modes.

That brings me to the most important point of the game. Halo 2 essentially defined online multiplayer for any console shooter. That is no longer the case, however. Halo 3 has brought a new definition for online battling to the table, and it rocks.

Delivers what, pizza? No. Single player and multiplayer fun.