Wow, this was good.
First, lets get this out of the way. The animation is a turn off initially. You think it is cheap, but it is actually akin to impressionist art. They are opening up options by moving away from realism. Even if you don't like the art style itself, you will start to notice the framing, the transitions, the surrealism, the switches in style. It becomes obvious that it wasn't done this way because it was easier. It is more that the animators chose to forgo realism because they wanted the options that a more limited animation style would open up. This is true of all animations of course, they are abstractions of reality, but this series takes it so much further.
The music was also quite good. I don't take much notice of music usually, but this was of high quality and well synched to the different moods of different scenes.
The characters are what really makes this a masterpiece. It is rare to see a series that charts the interactions and progress of four to six characters so well. Every single one of these characters is believable. Every single one could walk into the real world without seeming out of place. Every single one of them is interesting in their own right. Every single one, even the ones you sometimes don't like, is worthy of your attention. They make animated characters seem more complete, complex, and real than most live action performances. This is what animation is about.