I think the closing comments of IGN's review sum it up perfectly:
Mystic Heroes is simple and to-the-point arcade action. I like it for the immediate satisfaction. You can jump into the game and start testing out buttons and attacks without too much consequence. It also has a very old school flavor that I enjoy. Battling through hordes of enemies and then taking out a huge boss is pretty enjoyable. The catch, however, is that all this becomes pretty dull after a short time. It's too much of doing the same thing over and over. Multiplayer helps amend that a bit, but even then, chances are you and your friends will want to turn to something that has more depth.
I think Mystic Heroes is ultimately a title aimed at a younger demographic. If you're 8-14, you may find it quite captivating. I would liken it to a good action movie. There's no frills, just raw action. To other semi-interested players, I definitely recommend a rent if you want to invite some friends over to accompany you.
I haven't played the game, nor do I plan to. I also haven't heard of the PS2 version so I can't help with comments on that. Maybe some of our more PS2 savvy members (Fabrizo for example) can answer.
However, I think it needs to be mentioned that 3rd parties generally releases titles for demographics that they feal represent the core market of a console. I don't think that it is Nintendo asking them for titles designed for kids.
