I'm really not a fan of the wii but this game is by fay the best release for the console, house of the dead 2 & 3 were pretty good but had certain things missing due to the age of the original releases

So what's extra in house of the dead overkill? :

GTA style bad language
2 weeks later style zombies that move quick like their on crack
Combo bonuses with bonus multipliers for not missing shots
Great story line
And most importantly what this game lacked in the others, extra weapons and weapon upgrades
power ups that you can actually see(not hidden in boxes or barrels) and you have time to shoot

When you first start the game it plays a little slower compared to HOD 2 & 3, you really need to pick the right time to reload you gun because it takes ages, but weapon upgrades are available for each weapon including an upgrade to reload your gun faster

this game is a must for all wii owners