I'd say it's worth a buy. The multiplayer is fun when you have 4 players, and the single player levels arn't bad at all, not really repetitive. I still wish it was longer, and there are some issues with the crosshair jerking around on its own, but it can be fixed if you turn off rumble and the speaker on your Wiimote.
Still, if you're looking for a great Wii FPS experience, Metroid Prime 3 is the better way to go.