One of the best things about the downloadable games market is that it's letting a lot of hopelessly niche titles find an audience, particularly for the last year or so. Moon Diver fits that description like a glove, as it's a bizarre sort of spiritual sequel to the Strider series, made by the same director and with the same ridiculously frenetic action sequences.
The story is paper-thin and largely irrelevant. Here's what you need to know: it's after the apocalypse, you're a ninja, and there are a nearly infinite number of robots, mutants, and enemy soldiers that are between you and the giant boss creatures you need to destroy. Okay? Cool. Go to town.
Moon Diver is a four-player simultaneous side-scrolling action-platformer, and is best played at full-tilt. You get health and mana recharges by stringing together a ridiculous number of kills, so if you're not charging headlong into whatever the hell's to the right, you're probably doing it wrong. With that in mind, it's easy to be a total jackhole to your friends the same way you can in New Super Mario Brothers, by hogging all the power-ups, kill-stealing, and generally moving faster than they do. On paper, it sounds great, but in practice, this is the kind of game that destroys friendships.
Solo, the biggest problem I have is that the health meter's sometimes difficult to see. You level up quickly, and each time you do, you get a point to invest in your stats between stages. All four playable characters have a pretty small amount of HP to start, and how much damage I'd actually taken often seemed arbitrary. It wasn't helped by the frequency with which dead enemies drop health power-ups, so I was getting injured and recharging again within the space of seconds. Even if the meter was reduced to a percentage or something, it'd be easier to read than it is now.
Moon Diver feels like the kind of game that was released near the end of the Dreamcast's life cycle. It's aimed directly at people who grew up playing this kind of game, with very few token nods to the fact that time has passed between now and 1999. You can usefully compare it to most if not all of the good 2D games of the last ten years.