Mass Effect is a new intellectual property from famed developers Bioware. Mass Effect follows the epic journey of Commander Shepard, who is probably the most elite human soldier left. The Mass Effect universe could easily compare to one of Star Wars, or Battlestar Gallatica. Throughout the game you will looking for new party members to help take down the geth, Saren, and anything else that stands in humanities way.
This epic trilogy has been well supported by Bioware, as this game has two downloadable content packs available and Mass Effect 2 is already available for purchase and that game has several downloadable content packs available as well.
I believe this is the best the Xbox has to offer as a total package. Mass Effect features addicting WRPG game play with tons of quests, constant upgrading of guns and armor, extremely varied dialog choices, and persistent leveling. If you own an Xbox, you must go purchase this game immediately.