Best X-Box 360 Game Ever. Don't go by the Gamespot Rating!
The graphics are amazing, particularly so when playing the dialogs portions of the game. Facial expressions and body language are life-like, and a joy to watch. Where as in many dialogs-driven games, players often skip the dialog to get to the action, in Mass Effect, players will find themselves exploring every dialog option in each interaction just to watch how the characters interact.
A large part of these excellent dialog sequences is the voice acting itself. Every single character I've encountered in the game is convincing and interesting to talk to. The acting is carried out so well in fact, that you do not even think of the actors behind the characters at all, where as in poorly voice acted games, you can imagine the actor sitting at a microphone reading a script.
The ground combat is excellent as well, and must be approached much more tactically than your traditional FPS. Not only must you make the appropriate decisions before the battle, such as using the right weapons and ammunition for the opponents, you must also make appropriate decisions during the battle, such as when to use cover, when to advance on your enemy's ground, and when to use special biotic abilities.
There is a lot to do in Mass Effect, and it has great replay value. In addition to the main story and side quests, the game has free-roaming space and planetary exploration (ala the classic game Starflight) where you can survey for minerals and explore the planets surface for artifacts. Also, because you have very limited control of your party members, to truly experience all the character abilities in the game, you will probably have to play through the game more then once.
All in all, this is a wonderful game play experience that you will want to savor again and again and remember for years to come.