If you have played the other light gun-ish shooters on the Wii, then this one won't impress you at all.
House of the Dead: Overkill is an update to the old arcade House of the Dead games. As such, it seems like it was made just to be better than those games, and not competitive with other home console games on the market. The graphics are certainly better than any House of the Dead game and for that matter, they are pretty good for the Wii. The grainy "low budget 1970s horror film" look gives the game a lot of style and do a good job of disguising how technologically inferior the visuals are.
Other than the game's style though, there isn't much else good to say about it. You select one weapon at the beginning of each level and you can only use that weapon for that level. You have to unlock or "buy" weapons to use new ones. Since it takes quite a bit of money to buy a new weapon, you probably won't unlock more than one or two guns if you play once through. This makes the shooting action very repetitive. It doesn't help that there is hardly any variety to the enemies or how they attack you either. There is very little bonus material to shoot at in each level to make it interesting besides health power-ups and little green blobs that temporarily slow down the game. The settings might change, but the action feels like doing the same thing over and over again. At the end of each level is a boss battle, which provides a little bit of variety but isn't anything to write home about. Just playing once through this short game (it takes just a few hours) will probably leave you feeling like you have had more than enough.
The game makes a sorry attempt at adding a humorous story by pairing a "Man in Black" government agent with a profanity-spewing Samuel L Jackson type of character. Most of the sentences in the poorly-written dialog include some kind of profanity. It's not bad because it's offensive. It's bad because it is so gratuitous that it becomes mind-numbing by the end of the second level. The story doesn't add any value to a game that struggles to achieve mediocrity.
"Link's Crossbow Training" has a huge variety of levels that provide a variety of challenges designed to keep you coming back for more. "Dead Space: Extraction" is an excellent translation of the horror series onto the Wii and it has great shooting action unique to this kind of game. "House of the Dead: Overkill" is inferior to those games when it comes to the amount of the content that it has and the quality. The game is mediocre at best, and not worth a rental.