Serious Sam is probably the definitive Co-op frantic-shooter experience, the same way Unreal Tournament is the definitive deathmatch frantic-shooter experience. It's really a simple game, with just enough plot to make you chuckle before giving you a gun and telling you to shoot things, and if that sounds lackluster to you, you're missing the beautiful simplicity of it all. This game has no story, because it's not trying to have a story. It's just trying to make you shoot things, and you do that a lot. Single player is fine, and plays a lot like similar games (like Painkiller and whatnot), but if you're buying it for the single player, you're missing out. The place this game really shines is when you're playing with at least one other person at your side (but it gets increasingly better the more people there are). It's hard to describe exactly why it's so much better, but there's just something about running circles around a group of rampaging skeletons to cluster them together, as your friend goads a charging yak to burst through the enemies you've just corralled. If you want a game with a story, or one that makes any sense at all, this is not the game for you, and if you're planning on just playing the single-player, grab Painkiller or something instead. If, however, you think the only thing that makes frantic bullet-spasms and skyscraper-sized monstrosities better is sharing them with a friend, this is the game for you.
