Depressingly bad.
Where to begin with this giant disappointment?
Well for one thing the music gets so repetitive it isn't even funny. At first you think "Hey, this is some decent techno." But as it goes along, it becomes pretty annoying.
The sound effects I really can't complain with. Just your basic beat 'em up sounds of punching, roaring, and the like. However one thing that I thought was kind of nifty is that they added Godzilla's classic roar sound to this game. But that's all the niftiness I could find of the three levels I stomached.
My next issue is the boss fights. They're repetitive as heck, and the boss patterns can be figured out so fast that it's just plain stupid. Except with the boss that drove me to almost snap my cartridge in half, Jet Jaguar. Jet Jaguar was one of my favorite characters in the Godzilla universe, but here they just screwed him up BAD. Well for one thing, his pattern can barely be avoided, as Mothra or Godzilla, or whoever you choose to use, unless you're crunched up in the corner and even then you can't even hurt him without using a beam, which needs charging, and he attacks so fast that you could probably barely charge it at all before he hits you and takes the entire thing away.
But enough of my angry ranting on Jet Jaguar. Now, this game is a sidescrolling beat or shoot-'em-up. Depending on what screen you play on.
Yes, you can play as two different characters, much like in the original NES version, except there are several characters you can unlock in this one, such as Destroyah and several others. On the top screen you have your flying character, such as Mothra. On the bottom screen your walking man, Godzilla for example. If you ever play this terrible game, I recommend you play as Mothra or any of the top screen characters and simply mash the X button.
Another thing that really bothers me is the level length and how repetitive and boring they get.
Basically each level is the exact same thing, with, at every midpoint, a building for you to press three buttons several times at. Why press three buttons several times? Because you are forced, yes, forced, it isn't optional or anything like that, to destroy a building by pressing those three buttons several times.
In conclusion, you can do a lot worse than Godzilla DS, but I recommend you do yourself a giant favor and simply avoid this game. Pick up a good DS game like Castlevania: Portrait of Ruin for example.
Thanks for reading, and take care.