Japanese dramas always give us the weirdest plot which is not for everyone. But if you want to have a look at the philosophy of PolyA relationship, you could watch this. I personally have mix feelings. I can't say I'm happy with it 100% or I hate it 100%. But there are some points they did well and there are some points that make me believe it shouldn't exist.
1. The story was daring
Rather than solely featuring polyA relationship, it expands to what's considered "normal" and "not normal" as social norms. Therefore, there are a lot to say about its content, and it's up to each person's belief. My advice is that before you watch this series, form your own belief first and follow the story and then either adjust/ or stay firm to your belief, because if we do not have our own opinions, we would be swayed by them and feel lost in our own thoughts.
2. The second half was really heavy. I could watch Ep 1-5 in one go but I had to space out ep 6-10. The atmosphere in Misaki's apt is always very heavy, it makes us really feel that everyone in that apt is not happy. And this contrasts to a house full of laughters when Misaki is with Maki and Daiichi.
They also feature the patriarchal dad of Yuuki who brought a lot of tensions with his oppressive and obsolete mindset - which is very relevant in some families.
3. The first half is more about Shuhei - the homewrecker - came to Yuuki and Misaki life and stir it up, but the second half is more about Yuuki and Misaki dealing with their own problems. They all grow in some ways and find away to cope with it, although I believe after ep 10, they will have a lot of challenges coming up.
4. The series successfully whitewash Shuhei. It's really hard for us to hate him in series. No one in the series is to blame. They all need sympathy in some ways.
5. With the ending: [Misaki and Yuuki filed divorce but three of them continue to live together and try to figure out a life of 3 with multiple forms of "like"], I would say it is an Open Ending. They are somewhat happy but the fact that three of them wake up in the middle of the night also hints that there will be more challenges and problems await. But I would be more mad, if they end up letting Shuhei have physical rela with Misaki or Misaki falls in love with Shuhei. Ofc each persons would have their own ideal ending. Next, what they could do better is improving the chemistry between Yuuki and Shuhei. My gosh! I was waiting to see a "couple feeling" in the second half, but well still none. They really don't look like they love eo at all though I still feel chemistry in Yuuki - Misaki and Shuhei - Misaki. This is the point which I'm dissatisfied most.
Oh and the reason why it shouldn't exist is that, basically, this kind of mess starts from cheating, and I'm not comfortable with cheating. PolyA is fine but only if people in polyA consent to it and acknowledge it.