Have you read the manga? I'm not gonna say it's better or worse since it's pretty on par with the anime (plus it's been years since I read it). I think I got to like the final arc of the whole thing and then dropped it because
the final arc had to do with a set of sibling/twins (I'm pretty sure) that "intrude" on Mei and Yamato's relationship. Looked like love triangle shenanigans and there was already enough of that in the front half of the thing.
I'm gonna give a lot of leeway to Yamato, Aiko, and Megumi... because they're all teenagers. Yea, when I was in high school, I think I was pretty black and white with my feelings towards the characters. But looking back on it now, I kinda just feel sorry for them, not particularly Yamato though. Everything they've gone through is actually pretty traumatizing and the poor decisions and bad behavior, while generally reprehensible and manipulative, make sense. Aiko losing all that weight was also strangely propped up as an achievement when it's pretty devastating, physically and mentally, and it being something she really shouldn't have felt like she needed to do. And Megumi got wrapped in the limelight and had all this pressure on her to be one way. And Yamato's physical appearance became a part of that stunt. She pushed away real friends and felt like she had to buy people's affection. She didn't have a strong support system, she was living alone. And while her actions reflected her bad character they also reflected how desperate she was for positive attention. And Yamato was just a guy in his first serious relationship, stumbling around, trying to do the right thing but ultimately failing some of the time because most of what he went through was brand new to him.
I've seen Say, I Love You more than a few times and I really enjoyed it. It felt genuine and like the actions of the characters while at times arbitrary and also conflicting with their best interests, made sense since they were all teens trying to figure out the social complexities of interpersonal relationships. It was a good time and I'll probably watch the whole anime again in the future.