Great example of what Nintendo is capable of!
The gameplay. Yet another downside. The reticule in the middle of the screen is extremely inaccurate to control with the stylus. It provides wayyy too much leeway, making it so you had to meticulously poke and prod at the screen to aim correctly. And, sometimes, when selecting items, you'll miss and hit another just because of the touch screens inaccuracy, and end up getting killed. Especially in online mode.
The sound is yet another downside. It's extremely dull, and almost depressing. It feels like loneliness, when you're walking across mostly empty, ugly, dreary worlds, and having the music to match it, and only a few silly, primitive AI enemies coming toward you to shoot. The AI is another downside. It feels like you're playing a crappy spinoff for Doom.
There really aren't any upsides to this game but for the Gameboy idolizers who are amazed their "2-screened Gameboy" could produce 3D, and the "intuitiveness" (it's like trying to play Counterstrike with a touchpad, fergodsake!); I could go further into detail, but others already have. All in all, unless you already have a DS and you see this game in the bargain bin, I'd recommend not letting it tempt you.