Firstly, we need to address first is that this is a melodrama, and in that sense the drama was done perfectly. The color scheme was muted, the OST fit the mood, and the pacing was quite slow which melodramas seem to be known for. There were some scenes towards the end where that raw desperation was conveyed so well that it elicited a lot of emotions from me (aka I couldn't handle the feels). That said, the middle did drag a little bit, but not to the point where I lost interest. Everything was purposeful, and you can tell as the story progresses.
The story was quite new in the KDrama world, which I appreciated. And where else can you see the leads get married by the third episode? The first half of the drama was quite light, mostly centered on their romance while the second half had lots of angst and suspense. What this drama did well was weave the past, the present, and the future in which he traveled to into a clear and coherent storyline. Most of the show is actually foreshadowing, and finally in the last few episodes are the viewers exposed to what actually happened. It did build up suspense and intrigue, as you had no idea what would happen. It was unpredictable, but by doing that they lost a few people (and provoked lots of fan theories), but I promise if you stick with it, everything will build up to a satisfying end.
The acting was done really well on Lee Je Hoon's part. This was my first time watching any work of his and I was pleasantly surprised. I didn't particularly like Shin Min Ah's character, as sometimes I felt like she was unreasonable. Their chemistry was off the charts though, and perhaps because they were married early on in the series, we're given a ton of fanservice (aka steamy scenes). At first I was skeptical because the circumstances of which he married her wasn't the best, but soon you could really believe that they're in love. There isn't a moment where the music fades and the camera pans over their face in slow motion that tells you they've fallen for each other, but trust me you'll know.
I really loved the music. I think there were only about three main OSTs , significantly less than that of other dramas, but all of them fit the drama so well. They also played some beautiful piano instrumental tracks pretty often, which created a sort of gloomy mood befitting the storyline.
While I may not watch the entire drama over again, I would definitely rewatch some of the scenes.
Overall, I loved the way the story was told, despite the pace. I have to mention the color scheme again, because it was really genius and beautiful. Right off the bat, it helps you get into the solemn mood constantly reminding you of the overarching plot despite the cheesy happy scenes. I would recommend this drama for those who don't mind a slower storyline, because it's definitely worth the watch.