Good, but not for everyone.
You like shooting zombies? Really? Good. Do you think your money spent at those light-gun-shooter-arcades was all worth it? Nice. And you think it would be a nice idea to put together zombies and light-gun in a portable console? Great. That's a game for you. Be aware, what you'll get is good, but you won't get more than shooting zombies. Your character goes on his own, walking/running by some pre-determined paths, all the player does is to handle the shooting by tapping zombies or any other enemy around. After some shots, you need to reload, that is done by dragging an extra clipof ammo from the bottom-right corner to the bottom-left corner, where your current clip of ammo stays.
As we're talking about "touching, tapping, dragging", it's obvious that the action takes place on the lower screen, while the top screen shows your current health, lives and weapons. There are four weapons (handgun, shotgun, machine gun and crowbar), each one assigned to one of the d-pad buttons (or the face buttons, if you're left-handed). As the game goes on, you will get weapon upgrades, making your weapons more powerful and shooting/reloading faster. There's also the need to keep an eye for wooden crates and breakable glass, since shooting them gives you ammo and health. You can even choose your path by shooting a direction sign, but this isn't so frequent.
The story is worthless mentioning. To make short of it, you're some generic badass prisoner who wants to break through a sudden zombie infestation. Luckily, the story won't get in your way, so you can keep playing, that's more interesting. The graphics are considerably below of what a Nintendo Ds can do, blocky models and low-resolution textures will do bad for those who aren't used to "old school graphics" ("old-school" being used not in a exactly good sense).
Overall, the game difficult is just right. Some people complain about the reloading system, however not only this is more realistic (and in a good way, it really gives you the feeling of reloading), but it's also vital to keep the difficult from being too forgiving (since the challenge isn't too hard). You may need to restart your game once or twice, before getting the hang of it, but you'll do fine. As long as you don't expect more than mindless fun, you will notice that the game, in its simple ways, do a good job at that.