This is one of the most beautiful pieces of television I've seen in any genre.
From the start the series doesn't feel like any other bl/y series I've seen. The pace is very relaxed. Lead ins are long and cuts are often extended. The characters all have space to breathe, and we have the space to empathize with them. The series isn't littered with product placements, and yaoi tropes are kept to a minimum (partner feeding, wiping each other down, peeling shrimp etc.). There is very little of the standard Thai drama sound-effect punctuated drama-comedy dialogue. Important scenes often reach their crux in silence rather than against the background of a thai-pop crescendo. Overall the series feels more like a very well shot film than it does like a Thai drama series. It features a complex story, overcharged emotions (director New's hallmark), and touching romance. It's not a series for those in search of flesh however. Watching the series is an exercise in catharsis, not titillation. (Though I suspect that beyond the camera's eye that Ongsa is a pretty wild lover.)
The story is a romantic fantasy in which time travel is the fantasy element, embellished with magical butterflies and oracular shop-keepers. Time travel is the magic that the story uses to allow the characters to experience events and emotions that lie beyond what we experience in reality. This isn't a science fiction story. It's magical realism. Time travel here is some sort of spiritual miracle or miracle of fate. In fact I think that time travel is simply the way in which the author has chosen to tell an archetypal hero story. Ongsa and Suansoon are destined to be partners, but only if they can prove that their love is pure to the extent that they are willing to give up each other for each other's sake.
I suspect that the story's flirtation with causal loops (A causing B causing A) is going to upset some viewers, but it's pretty easy to accommodate them simply assuming some external cause to the whole loop. Call that fate or destiny, and it squares pretty well with the spiritual world of the story.
The acting is sound to phenomenal throughout, without dead lines or awkward sections of stiff manneristic acting. Tor Supakorn especially does an amazing job creating the role of young Ongsa. He's always very three dimensional, which makes it very easy to empathize with him. He manages emotional character arcs and ranges of emotions very well.
The key challenges to filming this story are making the various timelines immediately clear and motivating our empathy for the characters given their unusual predicament. Both challenges are met successfully. Changes in clothing, set dressing, and character demeanor help to differentiate the time-lines, and the use of flashbacks to tie the counterfactual regrets to something we can feel bad about serve to maintain the viewers empathy.
Overall I found the series very effective. It was easy to empathize with the characters and share in their emotions. It's very cathartic, especially at the emotional cruxes of the character arcs. I would add though that it's a story that needs to be enjoyed on it's own terms. If you try to watch it simply to follow the plot of story rather than letting the series engage you emotionally, it's probably going to seem a bit dull and repetitive with the various repetitions of events and flashbacks tying then and now together.