Please be warned, this game is NOT for everyone. Personally most hardcore shooter game's aren't for casual gamers. This game is quite fast paced and requires a lot of eye-hand co-ordination. I know some of my friends are horrible at that sort of thing so this might not be your game, if you can, hire it out first before buying.
Onto the good notes! The goal of the game is to shoot, avoid, like any other shooter. It makes very good use of the Wii's controls although the menus (there aren't many) would've been better to use the nunchuck to select things rather than the D-pad. The game is ultra fast paced and challenging, there are plenty of unlockables and i find it great.
This game is definitely worth buying if you're a fan of shooters, definitely worth looking at for the casual gamer, it can be highly enjoyable to some. Some will find it shallow and boring or too fast paced.