Since Castlevania Symphony of the Night the serie has become more like Super Metroid but with the typical Castlevania creatures.
Gameplay: Like mentioned it's almost exactly like Metroid. You're walking through different areas from one room to the other and slash down the enemies known of the serie: Bats, Succubus, Merman, Armors,etc. You can find different weapons like swords and spears and each weapon has its advantages and disadvantages. Besides that you can also find armors and accessories to increase the power of your character. But the weapons are not the only thing to beat down the enemies. You also get spirits from the enemies with many different abilities and these spirits are selectable every time and add something special to the game. The fights aren't very hard except some of the boss fights. To orientate yourself you've got a map like in Super Metroid.
Graphics/Areas: The graphic is very good for a GBA game and the enemies are well animated. Some of the enemies are cool and big (posted screenshot) and the different areas are truly different and some of them are just fantastic (posted screenshot).
Story: The story is about Soma Cruz who gets teleported into Dracula's Castle and fights against Dracula. But he's not alone and meets some people which are interesting and they tell you some interesting facts about the castle and Dracula.
Sound: The tunes are fantastic and match perfectly to the areas and creates a great one of a kind atmosphere like its paragon Metroid.
Conclusion: Aria of Sorrow is probably the best game for GBA with a great sound, good graphics, fantastic areas and a big role-playing part. The only negative point of the game are the easy fights. Now it's up to you which serie is better: Metroid or Castlevania? I think both are great series you just have to play.

Aria of Sorrow is a good Castlevania game indeed, but it is more like Metroid. Good review on it