I really feel a 5 is a good number for how I felt watching this second season.
I would have left this whole story (1st and 2nd season) happier if I just stopped at the 1st, tho I don't know if I regret watching it all, at least I can be angry with all the info haha.
A person that liked this season said some very valid points in favor of this season, so I would recommend reading some good reviews, because I do feel is a matter of perspective and taste.
I've read other reviews and I think they sum up pretty well what was wrong with it, but here's a short (and spoilery) review.

*****Spoilers ahead*****

Things I liked:
- Hye-Young is such a nice and lovely character, and he was respectful with Jojo all the time and that was great so see.
- Jojo's university friend is the very best (I wish we've gotten more of here).
- Gul-Mi's character, she grows so much, but doesn't stop being quite shallow (I love consistency, it wouldn't make much she changed her base personality, ***like other characters did***).
-Duk-gu's appearance was perfect, he stayed true to himself, and helped everyone he had set up to help.
- Sunho and Jojo's goodbye was pretty, it made me cry (and it was totally not enough).
- The acting was great, I've already watched Kim So-Hyun in "School 2015" and I enjoyed her, she did very well, and Song Kang is amazing to watch, I'll definitely be following him more. Go Min-Si, Jung Ga-Ram and Kim Si-Eun also did a very good job. Someone said this in other review from this page, and it made lot of sense to me, an I quote: "I must admit actors did a great job portraying their characters with what was given to them".

I didn't like (at all):
- The obvious and mayor complain is about Sunho, for me is not even like they changed him, he was just dropped and left stuck in the saddness:
-He is kind of forgotten mainly by Jojo, who runs from him every chance, never gives him an explanation tho' she always seems to care for him in a romantic way (that at the end was just her being confused, other review said that he was pressuring her when she clearly didn't love him, and on this I have 2 things to say, one is that I agree that he pressured her too much, which I didn't like, but I disagree with the part of she being clear, I felt she wasn't really clear until episode 4-5, and when she finally is clear, he backs away), he ends up figuring out why she left him all by himself (and thank's god he did) and then we're supposed to be satisfied with the convo at the end, something that may have happened if it took place a bunch of episodes back.
-I also felt that his friendship with Hye-Young was totally wasted, they fight at the start (or last season? I can't remember), and from that they're basically strangers, and I hated it so very much, they recovering their brotherhood would have been a beautiful addition to the story.
-On the other hand he himself never does an effort to get over the heartbreak until the very end, and when he does it's following the awful advice his mother gave him (basically fake it til' you make it), which is so sad and unfulfilling. He really deserved so much better than what the writers gave him and it makes me so mad.
- Suddently the mother cares in this season, when the last she didn't show any sings whatsoever of it (and as I said, gives him terrible advice).
-Talking about weird changes of personality, with no reason Jojo's aunt doesn't hate her at all, and she even likes here, and it makes no sense (I wasn't that bothered about it as I was with Sunho's mother cuz at least Jojo's aunt is a little helpful).
- Jojo was overall just a mess, and tho part of it makes sense (and that I liked), other part feels forced to make things happen, the writers changed her so much I barely remember how she was the 1st season.
- The thing with the spear and the fight it created between J and H was so weird and I felt missed the point with the real issues, like, he thinks she's lying bc she decided to make his alarm sound? Like... that's how life works... Idk, maybe it makes some sense, but with all the repertory of stuff he had to be angry, than the writers chose that is a waste. On the same like, I have a similar issue with Jojo in regards to Hye-Young as with Sunho, and it's the fact that she never gives any explanation for her actions, and that's an issue she also has in season 1, but as I always percived it as a main problem, I was hoping it was something she learned, but we only get a tiny glimpses of it, she never really explains the more important stuff to anyone, and that leaves the relationship between her and HY with an unfinished feeling to it.
- Overall it feels like the writers had a story they wanted to show, but it didn't fit the characters they had, so they just messed up the characters.