I'm a little confused about when both couples started falling for each other, but what the hey! I'm enjoying it so far. The pace is perfect. It's not too fast or too slow. I'm not even paying attention to how many parts there are because I'm enjoying every episode and the progression of the plot + characters. It's not draggy at all. I hope they keep it like that and end it two weeks from now, which would make it at 20 episodes? Dang. That's considered pretty long, but as I was saying, the plot is progressing well so the extra episodes will tie in everything well. I wouldn't mind if they ended it next week. As long as this week's episodes uncover most of the mystery.
Oh, and I'm happy that the n'rai disappeared! Thai's should really try getting rid of the clingy n'rais and just keep the main villains. It makes everything less annoying.