I'd take another 2-D masterpiece over a 3-D excursion any day. This game only affirms that belief.
The game takes place during WW2 (don't worry, you're nowhere near the war), and a castle not dissimilar to Dracula's appears. The church sends Jonathan Morris (a blood relative of the Belmonts, but he cannot use the whip to its full potential), a fighter in possesion of Vampire Killer, and Charlotte, a mage who aids him by casting devastating spells. You'll be able to switch between the two at the touch of a button, call out one to travel with you (or just launch a back-up attack), and initiate a deadly dual crush attack.
Gameplay wise, this game has all of the good elements from previous games, and introduces new ones. The dual crush is an attack where both characters initiate, and they vary between a holy ball of lighting to a summoning of all old Belmonts to strike at enemies. The spells that Charlotte uses take time to charge up, and can be charged more for an extra "oomph". Graphicly, this looks great, particuarly the backgrounds. None of them look stale, and the character motions are incredibly fluid. The only complaint in this division is that some of the weapons of Jonathan's (particuarly the normal swords) look exactly like eachother during an attack.
Sound wise, the game has everything a Castlevania fan should be happy with. The tunes are great. No complaints.
In terms of replay value, there are several quests to attempt, items to get, and different endings (only two endings, with the good ending having slight variations). There are two modes unlocked upon completion of the game, and a third available for killing 1000 Old Axe Armors. No, I am NOT kidding!
In short, this game will keep you busy for a LONG time. The hard modes are actually hard, and the game has plenty of reasons to keep it in your DS for a few weeks. If you are a fan, this is a must. If not, this is a good place to start.