I love it. In terms of combat system, it IS toned down compared to the prev Mana titles (less weapons, less abilities per weapons, less spells per element), and the story is irrelevent. You basically go exploring dungeons and dungeons again and again. You may find it repetitive. They're randomly generated in multiplayer, though. I'd say it's essentially Mana meets Diablo.
The music is nice, the graphics are nice (sprites and town are very cool, dungeon environments less so). Combat-wise, it IS a bit simple. I do quite enjoy the collision detection (lots of ricocheting around).
I love the game. I would hate it as a singleplayer game, but it is exquisite in coop. The flaws are very present, but I'm willing to overlook them because coop is oh-so-enjoyable. The downside is that every player needs their own cart.
If the thought of a somewhat simplified Mana dungeon crawl doesn't make you throw up, you might consider this. If the thought of using your sister as your primary weapon as you send her flying into the hordes of enemy with the hammer (which has a very strong ricochet effect) is appealing, strongly consider this.
My sister's not that great a gamer, so a simplified game in this case is a very good thing. This could be my favorite DS game for the coop alone. Again, despite the flaws.
Though, yes, heed caution.