The first three episodes are absolutely incredible, just so good. After episode 3, I was really thinking this would be an instant new favorite but the last two episodes personally let me down so I'm a bit sad. Episode 4 takes a turn I didn't see coming (you know, for the angst, it was to be expected) and sadly I thought the aftermath felt rushed. I honestly think the first three episodes set my expectations too high for the last two.
However, you absolutely do need to watch it. It's really gorgeous, both in terms of story and directing. There are so many incredibly beautiful scenes simply through the setting, the colours, the music or the meaningful silences. Oh-aew and Teh's chemistry is off-the-charts. I can easily get annoyed when there is too much focus on the main couple's other interests but they do a pretty decent job showing that no one can compete with what Oh-aew and Teh feel for each other.
That being said, I am ecstatic about "I Promised You The Moon". I'm also looking forward to seeing if doing a rewatch will change my mind on the last two episodes.