Going back to the soundtrack, after seeing the movie 4 times, I can tell you that the music is definitely noticeable and doesn't disappoint at all. And listening to the soundtrack itself, its amazing as usual.
Kylo Ren's theme is similar to Vader's actually. Every time he enters a scene, it plays, and is definitely a dark side character theme, no question. It's simple, but so is Vader's. I think I like it better than Vader's, just because I think Vader's is overused in the original films. Especially ESB. 
