A third of the way through this show, I don't really want to comment on the plot, but the acting is genuinely astounding.

So far this show has been very slow paced - it feels like the episodes up until episode 8 or 10 were just the set up; but for this same reason the characters are richly developed, and by the time the present day story kicks off we have well and truly fallen for the characters.

At first we see some clips of the present, but is mostly the backstory, in a slight chronological disarray. Aside from understanding the events that shape the characters lives, we get to know the characters well, even before anything has happened, and later on, (thanks the impressive synchrony of the actors, who not only resemble each other to a great degree physically, but in the mannerisms and expressions of their characters, creating the most uncanny portrayal of characters played by multiple actors I have ever seen) we see how they are effected by their lives, and also those parts of them that have stayed the same. Because of the slow pacing the introduction is very immersive, taking the time to explore details in the environment. There is actually quite a dense amount of information in the show, rather than having episodes full of fluff, but the show is quite thorough, and the character building is a major time consuming factor. Character building is important obviously, and is the key point of this drama, but it doesn't advance the plot much, which is why some may find it slow, especially in airing. I imagine in binging this however you wouldn't even notice the time going by, because as I said before, it's incredibly immersive, so you'll probably be completely absorbed in the flow.
Because the characters act so dependably like themselves in any given moment (rather than being directed by the story, and acting around the next plot point) the things we love about them are reinforced over and over again, and it creates a greater sympathy between the characters and the viewers, because, although in some ways some might say as people they are "too good to be true", with his righteousness and her optimism, they feel so much more real than many characters are written to be. As we know the characters so well, any changes or uncharacteristic actions are extremely apparent, and I look forward to any character development in this show.
A great deal of the credit for these incredible characters must go to the writers, who ensured their steady and reliable nature, but equally this result would not be possible without the collaboration of all four lead actors who work so well together (on that note can we give a hand to the casting dept.)

The story (so far) is relatively simple, unfortunate, and uncommon but believable. It's not revolutionary (boy meets girl, tragedy strikes, a decade later how are they dealing with the consequences when they meet again?), but it's not worn out either (I'm sorry for liking you back, the old man's a _psychopath_, and he might kill you / that's okay, I'll make it big on tv so call me / living life as a badass detective / famous now, but I'm traumatized, also you never called) - but even if it were, the important part is that it's well written enough to be irrelevant. We have the cliche of boy meets girl, but it's a romance, so I'm going to discount that one, as it's kind of like bolognese without tomatoes otherwise; aside from that, the characters are genuinely interesting and likable people from unique families leading everyday lives, and events are largely, if not entirely caused by other people in those lives - there is nothing cliche enough in this to make it predictable, honestly, it's hard to even say how the romance will work out, or if this is destined to end like some kind of greek tragedy. I think more than anything, so far this drama has been about two people living their very hardest, in spite of anything and everything that happens to and around them, and from here will be about how they will struggle to overcome the conflict of their unforgettable past and their unending feelings. There is a crime, and a degree of mystery and suspense, but while people get injured (as of yet) there's nothing particularly graphic.

The soundtrack is good, although the tracks are already recognisable; it reminds me of films like Black Beauty, A Place Among the Pines or A Series of Unfortunate Events. As for the rewatch value, I can't wait until next week to see our beloved Na Moo and Nak Won again, so I'm about to rewatch what's presently released again right now! (Look, just watch it already, 10/10 you won't regret it, and there's nothing else quite like it.)