The handheld Castlevania games are the true successors to Symphony of the Night - much moreso than all those PS2 games that got released. All six of these games are at least decent, if not great. If I had to rank them:
Castlevania: Aria of Sorrow +
Castlevania: Dawn of Sorrow (these two go together - the DS is obviously more refined but they're the same formula and they're awesome)
Castlevania: Portrait of Ruin
Castlevania: Order of Ecclesia
Castlevania: Circle of the Moon
Castlevania: Harmony of Dissonance (the last by a long shot - it's really a rough game...something about none of the Circle of the Moon staff worked on this so it seems more primitive)
Absolutely worth a play - it's amazing that Steam has all of this now. I have them all in original format so have not purchased, but if anyone doesn't this is a great way to get them now - it's a great price for a set of great games.