Almost interesting. Soul of Darkness is maybe one of the best games ever made for java phones, which doesn't exactly mean it's a great game, but it surely is technically impressive, from astonishing visuals, to interesting mechanics (but with mediocre combat) such as transforming into monsters, and using elemental magics.
I'd say the only aspect that is straight out bad, is sound design (or noise design, if you prefer), which I'd say it's probably a problem with the platform itself, because I'm sure that if Gameloft could, they'd definetely make something better, just like every other aspect of the game.
For some reason people compare this to Castlevania, which I don't really know why since it's just a linear beat'em up with vampires, and you don't even use a whip. Oh, and unlike Castlevania, this game has an actual insteresting plot.