Excellent, just like the others in the series.
THE GOOD:
Like SOTN, and the other 3 games before it, this game focuses on exploration, RPG like skill advancement, and action. Also, like the other games in the series, this game simply rocks. You explore the castle, gaining power ups to advance to new areas Metroid style. On top of that, the whole RPG aspect is here (leveling up, items, weapons, skills).
One great aspect of this game is the skill (spell) system. Every enemy in the game has a chance to drop it's soul. When you get the whole, you have the ability to use it's power. Getting every soul in the game, is a must for hardcore addicts.
The graphics are beautiful, the best yet for any castlevania game on a handheld system. The bosses, are, well, huge, and require strategy (using the right souls, items), instead of just hack and slash.
The music is also wonderful. There are many catchy (and classic!) tunes. When you obtain the best ending, you will unlock the sound test mode.
There are multiple endings like the other games in the series depending on your actions near the end of the game.
The Duel screen things is a godsend. Seeing the map or the status screen at the same time you play is wonderful. You will never want to play a game like this again without the duel screen after playing this.
THE NOT AS GOOD:
There is not much bad about this game. It is very similar to the other games, with nothing really newly groundbreaking to offer, not that it's a bad thing, why mess with an awesome formula? Still, someone wanting more will get more of the game instead.
The game does not really utilize the DS stylus. At the end of boss fights, you will have to perform a stylus move to win, and it is used in a couple other aspects of gameplay, but you will play most of the game without it.
OVERALL:
Excellent game. If you love the others, you will love this one. If you can't get enough of games like this, buy it now, then wait for the next one! =)