Long time no see! Anywho, I'd say that the initial length can run you about 12-15 hours. Pretty modest, but with items to collect, maps to explore, and levels to gain, the replay value is extremely high (I'm on my third playthrough ).
As for whether this is the best DS Castlevania so far, I'd say that I'm enjoying Ecclesia more so than either Dawn or Portrait. Dawn is definitely a pure Metroid-vania experience, while Ecclesia is more straightforward when it comes to exploration. Still, the villagers and different maps make Ecclesia much more varied, and therefore more compelling in my opinion.
As for difficulty, yes, this is the hardest Castlevania I've played in a while. The bosses and baddies rely on exploiting patterns and weaknesses more than ever. Some of the bosses can be very difficult. I even had to go to YouTube for tips after being defeated time and time again. It's that tough, but it's so satisfying.
Glad you're enjoying it. The music is definitely one of it's top points. I think this title is going into my top 5 DS titles as well.