Before started watching I have the feeling that this drama is overrated, but I wanted to give it a chance because of the premise. Unfortunately, Twinkling Watermelon was not up to the expectation thanks to several issues that I will explain later.

Overall, this drama was a mix of "good visual, make-believe, cute moments" with "draggy, non important, filler moments".

With the long duration of each episode (to me 1 hour is long LOL), people tend to remember the former more than the latter, creating a wrong perception that it was a decent drama while it's actually not. Perhaps this drama could only be considered good after we remove all the unnecessary? Or maybe it will be good if the following issues can be handled better by the production team, and then only we can overlook the filler moments;

1. Failed to focus on small details
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This is the major issue in this drama. The writer was lazy to take care of the small details resulting in many afterthoughts writing "cover up". There were also some questionable illogical actions from the characters that felt unnatural throughout the drama. The representation of deaf people was more on the superficially dramatic side instead of the believable one.

2. Non consistency in main character
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The main character, Eun Gyeol sometimes was portrayed as smart, responsible, and genius. But at other times he was portrayed as plain stupid. Because of that we had to deal with unnecessarily TIRING misunderstanding between him and his teenage father, Yi Chan for like half of the duration of the drama in each episode. I understand the writer wanted to create the so called cute moments and pseudo bromance but it felt repetitive and too forceful (and tiring). His slow ability to read the clues left by Eun Yu was another example of his character stupidity, where the writer used this to drag out the plot to the max.

3. Disappointing music/band element
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The other major disappointment in Twinkling Watermelon is its music direction. Some may say this is not the main focus of the drama. But please remember that the whole thing start and end because of music. From the young Eun Gyeol learning the guitar until he joined SPINE9.

Firstly, they superficially made Eun Gyeol as the genius guitarist. However it wasn't clear what was his style of playing and what kind of music genre he was into. Showing some random guitar techniques and Canon Rock did not help.

Secondly, the whole band-forming arc in the 90's was a JOKE! They confused making a band with making a sport team, where you need to find the most powerful player...LOL. It would have been great if they could show what kind of music Yi Chan listened to, rather than randomly picked Nirvana but didn't show anything close to that genre/sound in the end.

Thirdly, the choices of music, sound, and styling for the band was not ROCK enough for a band that's formed in the 90's. Similarly the background music selection did not vibe the 90's. Considering good budget the Korean have for their drama it won't hurt paying royalty to appropriate western songs if needed.

Finally, the band members were shown as doing all sort of things except playing music. It was not until Ep10 that we see them play and practice together. Even so it was like magic that everyone could blend in so quickly and able to play like professional backup musician at one try.

4. Pseudo 90's portrayal
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I guess 90's era is one of the selling point of the drama but the writer and production failed to utilize it. I didn't get the nostalgic feeling even though I was there (not in Korea though LOL). Nothing portrayed in the drama was really a fair representation of the era except for maybe some posters and vinyl album covers at random places. The technology limitation did not play an important role to the plot at all. Instead, the writer chose to overuse PAGER as one size fits all solution for the writing, which is a bummer since it was not a must have device unlike smartphone of today.

5. Pacing
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Most people found this drama boring during the beginning right up to Ep4 before it turned into better. For me, Ep1 was okay but it started become draggy and overstretch right after that. Only the last 6 episodes were showing some progress, with the following breakdown:

Ep11 - Ep12: They show more of the band and its members in action
Ep13 - Ep14: They show more of the romance
Ep15 - Ep16: They show MC solve the mission and conclusion

Even though many found the ending rushed, I personally liked it because they didn't explain everything and left it for interpretation. It was an ideal ending for this kind of drama IMO, albeit typical and predictable. Or maybe I just wanted to finish the drama quickly IDK...LOL