The story of Moon Diver could be summarized as "Move right, kill stuff, don't die." There are some anime stills with ridiculous lines such as, "All flights of fancy turn to reality," and "Beautiful. Worthless Humans. Kill them all." I'll admit, these are the opening lines. I skipped or forgot the cinematics after those gems. The twelve stages are titled thing like: "Day of Destruction," "Closed Paradise," and "Hades Opens." You will not be getting your literary fix from this game.
As a single player experience, Moon Diver begins to get difficult around the third stage. If you chain kill 50 enemies, you HP and MP are restored. Some enemy attacks sap your hit points quickly. If you die in a solo run, it's back to the stage select screen for you. The levels aren't too long, but it's frustrating to have to trek back through an entire stage just to be taken out by some boss's instant death attack. But you didn't come here for a single player review, did you?
Co-op is most definitely where it's at. Moon Diver is a much more enjoyable experience with a few friends. The game supports online co-op as well as local co-op. Unfortunately, it's either one or the other. You can't couch co-op with a friend and play with a couple of other pals across town. You can, however, play with four local players on a system. This is how Moon Diver was meant to be played. (Four players with supported voice chat is also good.)
Boss fights are a chore in single player, cake in co-op.
Whenever experience points and loot is involved, the first thing players want to know is, "Who gets what?" I'll break it down for you. In an online game, story progress only goes to the host player. So if you play Stage Eight online, but have only made it to Stage Three in your own game, you'll still only be able to select Stages One, Two, and Three in your game when your online session is over. Joining players keep whatever experience they earn, along with whatever MoonSault Combinations they pick up. Yes, it is a mad dash between players to pick up the little golden orbs that represent new MC's. It seems that some MC's are in fixed locations, while others only appear when certain requirements are met. If a player already has a particular MC, they can still pick it up. Don't be a jerk by double dipping.
In local co-op things are a little different. Since the PS3 only allows one player to sign in at a time, everything goes to Player One. For example: Player One plays as Red Guy and Player Two is Blue Guy. They both level up to level 10. Player Two is actually Player One's Blue Guy. If Player One comes back solo, he will have a leveled up Red and Blue Moon Diver. If Player Two signs in later, she will have jack crap. So any local players are just leveling up the host player's roster of Moon Divers. I don't know if this will be different on the Xbox 360 version of the game, since each player can sign in to a Gamertag. If it is, I'll update accordingly.