Definitely one of the best games on Xbox Live Arcade, and strangely not because of the graphics, sound or extras
Galaga isn't an overly complex game in terms of control. It simply involves side strafing to dodge incoming fighters and missiles while launching missiles of your own, but that's not to say the game isn't deep or involving. Figuring out the enemy attack patterns can get complex, and knowing when to sacrifice a life for the double fighter powerup is also important, such as during bonus rounds.
Taken as a whole, Galaga is a great way to enjoy classic '80s arcade goodness. If you are into that sort of thing, pick it up. You won't be disappointed.