I've only put in 2.5 hours, but it's a solid game. It's at least as good as the past several mediocre Mana games. . .possibly a bit better. It plays decently, and it's geared towards single player play. It's kinda easy, but I'm using it as a bedtime game, so it fits well.
Square's done a decent job of salvaging this franchise after its terrible start. 40 dollars is kinda pricey for a DS game, so I'd wait for a price drop here. At 30 and below, I'd give it a buy, with a caveat or two about liking kinda simple dungeon crawlers.