I'll write this review for those who have watched the Japanese version of One Million Stars Falling From the Sky. I was originally hesitant to pick this one up because even though the original story was pretty wonky, I'm a huge fan of Kimura Takuya and Sanma because they have such great comedic chemistry together (especially the Christmas and New Year's variety specials they do~) and 100MM Stars was really the starting point of their on-screen collaboration.
For those who aren't familiar with 100MM Stars, it was to Jdrama was Fashion King was to Kdrama. I'm the type of M fan that still loves them both, so don't trust my taste too much.

I usually can't stand watching Korean remakes because they tend to copy every~single~detail~ and invite endless comparisons, usually, for the worse. In this case, while the details more or less matched up the broader plot changed dramatically. The story is much tighter, fills in tons of plot holes, and is much kinder. For example, you get an explanation of why and how the two leads got together that actually made sense (gasp!).
That said, I couldn't help but remember the absolute wretchedness of the original at certain key points—I think a lot of scenes that should have totally fallen flat for the Korean-only viewers had me practically in tears, and vice-versa. So don't take this review as reliable if you don't have the BG of 100MM Stars.

The Smile Has Left Your Eyes was a very pleasant surprise. I accidentally started watching because of Seo In Guk (The titles are different! The story starts out differently! How could I have known what I was getting into??). He brought a whole new image to the MC. Rather than Kimura's short-tempered abrasiveness, Seo channeled more of a cool-headed psychopath type of character. It was only until quite late in the series that he really started to overlap with Kimura's style/mannerisms and I really began to miss Kimura deeply (at a certain angle, in those same clothes, with that hair color...). I actually really like the MC's new job working in a brewery (less comparisons to SxS's champion cook!) and being able to see his interactions with his coworkers opened up a new dimension to the character. Seo eventually evolves into a softer, more self-assured version of the MC that is pleasing to the eye.

Seo's scenes with Park Sung Woong really couldn't match up to Kimura-Sanma's—the vicious but clever back-and-forth banter, the overwhelming tension in their relationship—all of that is gone. There was much less interaction between these two characters, and it boiled down to one simple flow (distrust -> trust layered with guilt as Park learned more about Seo's background — compared to Sanma never trusting Kimura, but gradually coming to pity him as their relationship constantly flips back and forth). That said, if you're not used to watching makjang Kdramas some of Park's interactions with Seo come off as absolutely ridiculous; maybe it was an effort at "localization". I was deeply unimpressed with the older brother character, despite adoring Park as an actor.

To fill in the duo's missing scenes, we get Jang Young Nam as a brand-new character starring as Park's buddy-cop and she really does a great job of driving Park's role to its depths. She's a strong, confident and lively breath of fresh air among all the testosterone and provides a healthy balance against the head-smashing vapidness of the young ladies chasing Seo. Her presence was deeply appreciated, not least because 100MM Stars felt like an anti-women anthem.

Speaking of, Jung So Min as the female lead had great presence, strong acting (better than Seo), and was all-around an improvement on Fukatsu Eri's character. She single-handedly delivered so many scenes; without her, this remake would have collapsed. The script dragged her back at times, but she delivered with sensitivity and talent.

Smile cut out a lot of empty filler from 100MM Stars and packed it with background plot (minus some emptier staring-down-the-street scenes that seem to be in fashion with Kdramas recently). Much, much better than hours of repeatedly being subjected to "boiling kettle flashbacks" and "handball tournament training". There was actually one scene where Seo played with that damned handball—I cried from the bottled-up repressed trauma, but I'm not sure what first-time viewers saw in it as the detail popped up out of nowhere.

Maybe it's because 100MM Stars really tore out my heart, but I appreciated Smile's shift to a happy ending (well... a happier ending). If you're still harboring damage in your heart because of 100MM Stars, I recommend this as your therapy. I'm not sure it stands up among the Kdramas greats.