At the time of writing, I've watched this series start-to-end three times, with some extra repeats of my favorite scenes.
This was such a phenomal show that I'm truly struggling for words to describe everything I'm thinking and feeling.

To deal with the easier items first:

1) Music: Fantastic. Mix of English and Korean songs. Poignant when needed, and hyped up when required. The cherry on top of an amazing show.

2) Acting: Top notch. Each and every actor brought their character to life. I believed them all. Kudos, congrats, and THANK YOU!!

3) Cinematography: In a word? Beautiful. Watch it and see for yourself ;)

4) Story.....

So!
This is where I hit the -.50 in my rating.
The first few episodes leave you interpreting a lot of what you see.
You need to guess, infer, and make intuitive leaps. Normally that's fine, we don't need to be spoon-fed everything, but in this case the show took it a bit too far. Making the audience think is one thing, but leaving them confused is another. And while the confusion did not turn me off the story, (I pushed through it), I'd have much rather preferred some clarity over making my own conclusions - which I was forced to do in order to continue watching, otherwise nothing would've made sense. (Meaning, you can't leave those moments hanging in the air; you need to come to some conclusion in your mind regarding what they mean and how they happened because otherwise the story literally can't be continued.)
That's my one and only complaint on the entire show, and my reason for the 9.5 rating.

As for everything else?

OH MY GAWWWWWWDDDDD!!!!!!!!!!!!
Can you hear my screaming? Because I am!!!! That's how amazing, terrific, insightful, and engaging this story is.

The connection between the two ml's is on another level, physically and emotionally.
The subtlety of the acting- a glance, a fleeting touch...wow...
The nuances of the characters, what they think, how they behave, what they chose to do and why...I've never seen anything like it and I'm addicted!!! Storytelling at its finest.
What's your cup of tea? Emotional connection? Physical intimacy? Well, you get them both in spades.

I don't think I'll ever get over this story, or ever get bored of watching it again.

Okay, so...let me see if I can summarize the story itself.
(**Spoilers ahead!!**)

1) A rich, popular senior dealing with depression and long-buried trauma.
Family pressure issues.
Cheating ex-girlfriend.
A jealous 'frenemy' whom I wanted to strangle through my screen on multiple occasions.
Meets a shy freshman and takes him under his wing.
Feelings develop.
Tragedy occurs, leading to trauma being triggered.
Major free-fall.
Avoidance + relationship breakdown.
Healing.
Reconciliation.

2) Painfully shy and introverted freshman.
From a small town, dropped into the vast ocean of Seoul, struggling to find his way.
Approached by a kind senior and falls at first sight? Fascinated at first sight? IDK, watch and draw your own conclusion lol.
Starts feeling comfortable, happy, safe, around Senior.
Catches feelings.
Tragedy occurs, leading to an unwanted relationship breakdown.
Steadfast with silent support and active encouragement throughout time apart. Respectful of boundaries, and patient. Consistent with support.
Reconciliation. Communication. 'Happy for now' / HEA ending.

3) I love the friends. Shout out to Ae Ri and Joon Pyo.
I hate Park Tae Hyung. It's one thing to appreciate honesty after dealing with two-facers but this guy is an entirely different matter. You should expect basic respect and support from your friends. See Yoon Won. This guy ain't it. (Watch to understand my rant lol).

4) The evil ex-girlfriend...a bit over-the-top sometimes. But not too much/often? Idk...I lived through those moments without rolling my eyes too much so I'll rate it "ok".

Which leads me to the combined acting + storyline.
Nothing was overdone or overdramatized.
You know how you can watch a show where everything is a caricature ? Either the acting is overdone (no one has those expressions or reactions in real life) or the story is overly dramatic (ie completely unrealistic)?
Well, aside from the ex-gf on a few (minor) occasions, this show was perfect in its execution of reality. Acting was real. Story was realistic. This could happen IRL. Everything was so natural. The flow was amazing.

Overall, a masterpiece.

If you've read this far, congrats and thank you !! LOL.
Now go watch the show (pleaseeeee!!!)!!!!! It's fantastic and you won't regret it <3