It's not revolutionary by any means, but Onslaught provides a healthy dose of arcade-style fun, and great WiFi play
Gameplay--Simply put, this is sort of like an arcade shooter, but you get to control your character and you don't have to put in coins to keep playing. There's four main weapon types (pistol, shotgun, machine gun, and rocket launcher) which are selected using the control pad. Upgraded weapons are rewarded for getting a good completion grade on missions, and some you just find during a level. The nunchuck control stick, obviously, controls your character, and it's also home to the coolest weapon in the game--the lazer whip. Holding Z and swinging the nunchuck will allow you to swing a badass lazer whip at your enemies--which is great for close combat. Holding the C Button on the nunchuck will let you pull out a cool looking grenade, which is thrown by swinging the nunchuck. Like most Wii Shooters you'll use the pointer for aiming, which once again shows why the Wii should really be the main console for FPS'.
Multiplayer-- Really great. It's online only, but it's easily the best online WiiWare offering yet. You'll be pitted against up to four other players (though you can choose to start the game with as little as two players) in a competition to see who can kill the most bug-like monsters within the given time limit. It's very simple, but very, very fun and there's tons of multiplayer maps and you can use your upgraded weapons from singleplayer mode online as well. You can choose to do a free battle, or a ranked battle (where you'll gain points to add to your running online score. Quite similar to VR points in Mario Kart Wii.). There are also online rankings to keep things competitive.
Closing Comments--Though the story is laughably bad, and the writing is nothing short of rediculous, Onslaught has it where it counts--fun. You'll get sick of hearing your team-mates yelling the same phrases over and over ("That's why you're still a kid!" is the worst) but in terms of overall fun-factor and great online multiplayer, Onslaught is one of the best WiiWare games yet. For 1000 Wii Points, the game is totally worth it. Let's hope we see more FPS on WiiWare; and on the Wii in general.