Overall:
Galaga is a really awesome game, it's one of the original space shooting games, it offers much fun, and it will give you a fair challenge.
This game is honestly my second favorite space shooter out there, I've had lots of fun playing it, although I have ragequit the game several times due to being at a high level several times and lost due to losing my nerve when getting to the high levels and I would die.
The game has great game play, which makes it addicting, and even though you may die, you just want to pick up the controller again and keep playing because you think that you can beat your high score this time.

Graphics:
Galaga for the NES has pretty much the same quality of graphics as all the other quality games on the NES at that time. The only problem is that I didn't really see much of a difference in the levels, or the enemies.

Sound:
The sound in Galaga is of course 8-bit, so it's not all that great sounding to your ears, but the sound effects are pretty good for a NES game, the crashes sound like crashes, the shooting sounds like shooting, and the theme is pretty catchy!

Addictiveness:
This game is really fun to play, and for me a game that is fun to play is addicting. Arcade games were meant to be addicting, and this game was originally an actual arcade game.
I could play this game for quite a few hours before I get really bored.

Story:
Sadly there isn't any kind of story to this game, you're just fighting aliens in space in you little space ship, more than likely trying to save Earth from an invasion.

Depth:
Although all the levels looks identical, you can pretty much play this game forever until you lose all of your lives. Some people have gotten extremely high score, which they must have been at least past state 50 to get. I'm this game has quite a bit of depth to it.

Difficulty:
As I said before, I'm not the best at this game... so I think it's fairly hard, although it's not the absolute hardest game out there, the game certainly gets a lot harder once you start climbing up the levels, and sometimes(at least to me) it will put you on the edge and get kind of nerve wrecking.
Nevertheless, Galaga will give you a challenge, especially once you start climbing levels.