I listened to the OST yesterday, and I must admit it - I'm impressed. It's melancholic, dark, mysterious; it perfectly fits the series.
One of my favorite songs is
Drift Mind, Monster's theme. And I love David Sylvian's
For the Love of Life - it pratically sums up the entire series.
Also, I was surprised they decided to release Monster OST so early. We got songs that'll play so far ahead of the anime, like...