This was one of very few anime that I could definitely watch all the episodes in a row and not even feel time passing. And I will definitely watch it again sometime. Despite all the negative reviews, I felt the story flowed so nicely that as soon as the ending song started I frequently caught myself in surprise thinking "what? already?".
Main themes are contrasts, knowledge/ignorance, dreams/reality. Inside the narrative, though, there is a lot of refreshingly progressive turns. There are strong, feisty intelligent girls interested in sex, there are strong emotional relationships regardless of gender and time, there are people in sync with animals, and there is clear character development. I was hooked, and I can't wait to read the manga.
I thoroughly enjoyed Psychopass and Guilty Crown before landing on No.6, and I do think it is a good series for any fan of dystopian sci-fi.