Castlevania: COTM is an amazing game!
The graphics are good for one of the first GBA titles. It's certainly not an ugly game! Your character can sometimes look pretty stiff but the monsters are all well animated.
The sound is good but it's nowhere near SOTN's musical scores.
The gameplay is largely the same as in SOTN. You get to kill monsters, explore every corner of a castle, gain levels and weapons. Nice touches like having cards you can equip that gives you certain attributes is cool.
The quest is pretty short considering i was able to beat it under 9 hrs. The game does give you reasons to come back and search for secrets items you may have overlooked. The fact that you can take this game anywhere is also a great thing!
In conclusion if you like Castlevania you need to check this game out!