This is, without a doubt, the best light gun game that I can think of... There is a plot, and it is actually good. The cut scenes that advance the plot are really good as well - well written, good voice acting, and they're really funny. The graphics are great, the game is fun, the controls are good as well.
If there is anything negative about the game, it would be the frame rate... Occasionally (luckily it rarely happens) when a group of zombies swarms you, the game will slow down. This can result in you missing a shot, getting hit when you shouldn't have, etc... but it is only a minor problem as it doesn't happen often in the game. The only other thing I could want from this game would be the ability to change the button setup in the controls - but again, that's a really minor inconvenience in an otherwise great game.
WARNING - This game is not for kids... not to be played while kids are around.. and not to really even be discussed while kids are around. This game has been made along the style of the old 70's grindhouse horror films - complete with over the top violence, gore, sexual situations, cursing, and outright gross content. Some people have actually let the content affect their review score, but it's kinid of the entire point of the game - to let you play a 70's grindhouse zombie film.. with all the sexx, cursing, and violence that those films had. You'll get a good idea of this from the minute you put the disk in, as most of the songs include cursing and off color humor (one song has a character saying "Don't call me your dad... I'm just the guy who's f#$ing your mother".. and another song is about a characters sexual feelings toward his mother). The opening cutscene in the game starts with one of the characters yelling "Whats up mother f!" - very first words in the game. So it's definitely not for kids.
Beyond that, fun game. Great replay value due to upgradeable weapons, unlockable features, and "directors cut" levels.