Soul of Darkness is a Castlevania knock-off and doesn't hide it. The main character, who looks like a Belmont, has to travel to a castle in order to fight a vampire and save his lady. Even the title sounds like a Castlevania subtitle. So is this $5 downloadable game good enough for someone craving a quick Castlevania fix? It depends on what you're looking for.
If you're someone who places a lot of importance of the presentational aspects of a game (story, graphics, music), you might find the game satisfying. Graphics-wise, the style apes the Castlevania style perfectly, and the quality is almost there. Other than having to deal with a strangely low frame rate when the "camera" is panning, I don't have any complaints. Music-wise, this game doesn't come anywhere close to its inspiration, but really, did you expect to? The midi samples are not of a high quality, and the compositions sound a little bare. Story-wise... well I've never played these games for their story, but it seemed to me perfectly in line with a normal Castlevania story.
If you play the Castlevania games for their gameplay, then there are a number of things that might take away your enjoyment. For one, this game doesn't have one vast connected map, Metroid-style. Instead, the game is divided into 10 levels. And if you think that means you'll face the same challenge that the "old-school" Castlevania games presented, think again: you have infinite lives, and the respawn points are plentiful, making death in this game pretty much meaningless.
As far as platforming goes, the game doesn't ask of you to make too many difficult jumps, which is probably a good thing given the problematic controls. For instance, there is a serious delay between the time you press the jump button and the moment your character will actually jump. Prepare to fall off ledges when you absolutely should not have very often.
The game does make some efforts to distinguish itself from its inspiration. For instance, without spoiling anything, not every level falls under the "platformer" genre. But ultimately, as I often find is the case in games that try to break the monotony by tacking on levels that play completely differently, those levels aren't all that satisfying, and feel undercooked.
The whole game takes maybe 2 hours to finish, and though the developer did try to offer some replay value by adding a Hard Mode, all this does is have your character take more damage with each hit, not exactly a compelling reason to go through the game again.
Ultimately, I think this game probably worked better as a cell phone game. You'd get an adequate Castlevania clone on a platform that offered no other choice, plus it is very casual and, being very short, doesn't feel like a "commitment" like some longer games might feel: you can play it, finish it quickly and the forget all about it. But on the DS, the game can't help but suffer from the comparison: you'd do much better by hunting down a Castlevania game on the DS, any of them, or simply replaying one you already own.