A good peice of gameing history and a fun if you have pals over, but it's really short lived.
Story: Well, in this type of game a story isn't really needed or cared about. Your a guy with a unit of people who have riduclously cartoony weaponary and go out to kill bad guys. Period.
Gameplay: Super challenging side scrolling action. You will die several times and you will distroy everything in your path like a madman. You can pike up all kinds of weapons and ride crazy destructive vehicles. Unfortunatly, this adreniline rush only lasts a few hours, a little more if you have freinds, before the game kind of dies out due to the fact that without the risk of a "game over" kind of kills the intensity and halves replay value. It just doesn't last long enough.
Graphics: Old-school (think SNES) with the interesting art style of mixing serious weaponary with wacky cartoony spinnoffs. Think every nutty, if not cheesey, Sci-fi action movie you've ever seen and stuff it into run on action sequence and you got Metal Slug 4 and 5.
Sound: Again. Old. Interesting, but not really touched on it's way to the PS2.
In conclusion, if you ever watched or played with GI-Joes as a little kid or like war movies, this is the game for you. It goes by fast, but is very enjoyable.