What Xbox 360 games are really good and worth buying?

It depends on what you like really. I have to say Orange Box for the most overall value. Gears of War and Halo 3 are pretty good. Also, both Halo 3 and Call of Duty 4 have great online multiplayer. Mass Effect is a good RPG. Rock Band is good if you like to have real friends over, and DDR can be good for that too. Grand Theft Auto IV is great fun to just mess around in. For a family geared time, check out Viva Pinata (the original) or Lego Star Wars/Lego Indiana Jones