For those unfamiliar with Castlevania: it's an action/adventure/platformer, and this one also has some RPG elements, so you level up when you defeat enemies, equip items, and use the DDS system (by combining magic cards, you get spells and weapon enhancements).
So what's so great about it? Well, the game is very challenging, so you won't be finishing it in a hurry. It took me about 20 hours to get to the final boss.
Another great thing about the game is the music. I recommend using headphones for the best sound experience.
The graphics are actually quite good too, but they're hard to see (as with most launch games actually).
I was a casual Castlevania fan before I bougth this game (I played some of the NES and SNES games) , but this game got me hooked on the entire series. It was also the reason why I bought the GBA.
Conclusion: if you're looking for a challenging action/adventure/RPG, you can't go wrong with this one. One of the best Castlevania games too.