So here's the lowdown.
Reviewers from game sites like IGN or gamespot will tell you it's a 5 or a 6 out of 10. Most of these people haven't even played through the entire game, which is strongly evidenced by them making completely incorrect claims like "you can only equip 3 badges at a time," when you can, in fact, equip up to 5, but only after chapter 3. Indeed, these people hardly even played this game.
Which is understandable, and it brings me to the other side of the reviewers:
The die-hard mana fans are bitching and whining because it's not a "true mana sequel." They were expecting, as usual, the exact same gameplay dynamics that have been used in every single Mana game so far. This game is different, gameplay-wise, and the fanboys hate it for that. They would rather have more of the same, so they rate it low as well.
Not-so-die-hard fans of the series, though, can definitely appreciate what the game DOES have: interesting gameplay, an awesome physics engine, and a storyline that stays very, very true to the kind of things you'd expect from Mana games. It even has a few characters making appearances from Legend of Mana, like Watts, the beloved weapon's smith with the (and I quote from LoM) "great helm." The story really is fantastic, but I'll be honest, I'd almost lost hope. 1/2 way through chapter 3, I was worried that it was just going to be some little boring story with the same gameplay straight through, but the story really picked up at the end of the chapter.
So it you're going to play it, keep an open mind about the gameplay, and be prepared for a great story that does, in fact, stay true to the signature Mana style many of us have come to know and love.
Personally, I love it. The music's great, the voice-casting is very good (except for the main hero and villain, as usual in any Square game), the gameplay is rather fun (though people with OCD will hate it because they'll have to break every single breakable item when they don't really have to), and again, the story's great.
My only complaints are the map/radar system, which can be very vague at times, and the camera, which you do control, but sometimes it becomes a bit of a hassle. Plus there are a few odd things about it. Like shopping can only be done from the start game menu, outside of the actual game, and pets can't be used in the storyline. :\
Overall, I'd say it's about a 7.5/10. A decent game, though nothing spectacular. I doubt it's the death of the series, though I do wish they put a little more work into it.
Buy it if you're a big fan of the Mana storytelling, characters, and visual styles.
Rent it if you're the type who only cares about gameplay.