Personally, it is the best BL series that I have watched so far. That's why I gave it a overall rating of 10.
First of all, I'll like to say that the overall concept of the culture of hazing in SOTUS  is very well developed and portrayed. Some people say that they didn't find it to be a BL series until after the first few chapters, but I think that is what makes it so good. The transition from junior-senior relation hardships to the love affair seems very natural and the story treasures the portrayal of senior-junior relationship until the end. The fact that it centers on the SOTUS system was a great success and left many emotions on the viewer. I guess these are the main reasons why many international fans actually watched all the episodes on one night (including me), just because the storyline is very well developed and so sweet.
Of course, the success of the series can be well explained by the writing, which is  very well done by Bittersweet. It might have been very hard for her to transform a novel into a drama script, but everything was just on the point and I feel that she has full control of all the characters. All the side-characters are also very fun and interesting, because each of them have distinct characteristics, which is often hard to achieve because BL works often just focus on the main couple.  
There were some criticisms and quarrels regarding the casting, but they all did a great job at the end. A novel adaption always have those criticisms on how the actors don't really represent the character well (like Krist not looking like Arthit). Although, in the end, there are so many fans attracted to their chemistry and if it was not these two, SOTUS wouldn't have been highly rated as it is now (and vice versa, SingtoKrist wouldn't have been this popular if it wasn't for SOTUS). That's why I gave it a 9.5 for both ratings (the leftover 0.5 is left for my high expectations for SOTUS S).
About the music, it was GREAT. I usually don't really mind about the music used for dramas (unless it sucks as much as it disturbs the story), but is was the best-fit for this series. The song, "Soh Sut Soh Say" performed by Keng Thachaya, was recurring through the episodes, which is actually really risky because if the music is too catchy, it might disturb the actual scenes. Although, for this case, it enhanced the story so much. Its catchiness was a success, and it also made a good fit into the a bit scary hazing SOTUS atmosphere as well.
Moreover, I love Gun Achi's voice, and the song, "A Secret in Our Hearts", is very dramatic and fits into SOTUS so well.
At some scenes, the song is very LOUD, but its SO GOOD. For the climax scenes, the acapella version was played, and it portrays Arthit's feelings really really well (although I didn't understand the lyrics at first sight). I've downloaded the song right after I finished the series, and am listening to it everyday, literally everyday.
(I just imagined if it wasn't Gun, it might have been disturbing haha)
The instrumental versions were nicely incorporated too; the editing was nicely done with successful cuts and fade aways which did not hinder any of the scenes at all. I think this recurring use of the same song lead to the success as it is ingrained in the viewer's minds (in mine too), and could recall the series as soon as that upbeat music starts playing.
Well, all this leads to a rewatch value of 10, because...you know. I rewatched it for a hundred times like crazy.
If I would suggest an improvement for this series, maybe there could have been an additional episode (or one episode could have been used) for Arthit trying to find out his feelings more, because it was kind of sudden for him to speak to P'Tum and make up his mind.