As far as looks go, the game looks great. I mean, it's N64-era tech, but it's a handheld, so I can forgive that. Besides, I still play Doom, so it's not like pixelated graphics and non-smoothed textures bother me anyway. The controls are the real interesting thing. You use the d-pad to strafe and move forward and backward (like WASD or ESDF or whatever you use). The touch screen is used for freelook. Move the stylus up and you look up, down and you look down, right or left and you spin. Like a mouse. Also you can use the weird strap thing that comes with the DS for this. It's slightly more comfortable since the stylus is the size of a goddamn toothpick, but you lose accuracy since you need to touch parts of the screen to change ammo, activate visor mode and use the morph ball (the only one I have issues with is visor mode, since the button on the screen is so tiny).
This scheme is decidedly brilliant, and you can pick it up fairly easily if you have any FPS experience. Takes a bit longer to actually master it, but after a few seconds you probably won't have any problems with basic navigation. After a few minutes you'll be killing stuff with no issues. Jumping is done by double-tapping the screen or pressing one of the buttons (A, B, X, Y), though those are out of reach, so double-tapping is your only option. Firing is done with the left shoulder button, which you have easy access to, though the placement of the shoulder buttons on both the GBA (all models) and DS is somewhat awkward and uncomfortable.
The major issue is hand crampiness. Crampiness is a word I just made up. The DS not built like a normal controller with grips for holding with comfort and stuff. It's flat and there's little to rest your hands on if they're bigger than those of a six year old child. I find that playing for more than five minutes or so hurts my hands and I have to take a break. Also, since your right hand not at the edge of the unit like your left hand, the weight of the unit is off balance and one hand takes on the brunt of the weight. This might be fixed with the DS Lite, since it's lighter, but the smaller size of it will probably cancel that out.
I also played a couple wifi matches last night, and it was very fast paced and lots of fun. Sort of reminds me of Quake or Quake 3, only with less weapons and on a much smaller screen. The lame friend code system from Mario Kart remains in place (where each player needs to have the friend codes of everyone else playing in order to actually play each other), though it's streamlined quite a bit and you have the option of selecting which game you want to join instead of waiting for random opponents. There's also VOIP in the lobby which works surprisingly well. I was expecting crackly, barely audible noises, but it's quite crisp and clear.
Overall, this is about the best a FPS game is ever going to control without a keyboard and mouse combo (or keyboard and trackball if you're a communist), and it certainly seems to push the DS hardware as hard as possible. I can't comment on the total single player game just yet since I haven't played enough of it, but so far it's good fun, if not repetitive, so I don't think anyone will have problems with it. If you like action games and you own a DS, this is pretty much a must-have.