I've been playing this game a couple of hours, just beaten the first boss, and whilst the game is an interesting idea, and the co-operation between the two characters is mostly well executed, there are a few niggles that get in the way of really enjoying this game.
Firstly the player character, Tepeu, seems unwilling to respond to commands at short notice - want to run to the edge of a gap to jump across? Don't, because he gets to the edge and thinks to himself *well it's a bit late to tell me to do that, I'm going to just run off the edge*. Obviously after a while you get used to needing to jump a couple of seconds before you want to, but I expect better from a game where considerable effort has been put in to originality. The second thing is the fact the Majin (the big monster thing) is slow. Really, really slow. Lumbering around 100 yards behind you is normally fine, but waiting for him to catch up to co-op with him adds up - especially when you need him to advance the plot. I appreciate he's supposed to come across as slow and powerful (in fact he seems to be mentally 'slow' too), but when foes four times his size can dance around him there's no reason to have him lag behind so much.
Aside these two gripes the game is entertaining enough, and is certainly playable despite its flaws. Although Tepeu lacks personality and is easily forgotton, Majin has more than enough, and whilst the graphics are far from perfect (Tepeu again feels like an afterthought), the story is attractive. Worth a play, if the price is right, but never going to be a classic, and I can't imagine it having great re-play value. If you didn't enjoy the demo, then avoid completely, it doesn't get any better.