ML and FL are more like SML and SFL (especially anti hero-ish ML) and kind hearted SML and positive lovely but poor SFL are more like ML and FL. Plus all 4 main leads have almost equal screen time (though FL maybe has more and except of 4 lead's screen time there are lots of scenes/story line of FL's family). Even romance of second couple looks more like main romance, on the other hand main couples relationship is somehow cold and uncertain. I would say in this drama 4 leads are equally mains.

As everyone mentions, second couple as characters and as romance are better, though I still have interest in main couple, because 1. I like ML actor 2. I'm interested in ML as character, we still don't know what kind of person he is, what's his goal and what he really feels about his close people. Though I can't see chemistry in main couple and will be bored/skip after story will be clear.

one of best thing in this drama - it doesn't have primitive cliche dialogues, silly misunderstandings and stupid characters who can't see/think/understand obvious things and doesn't have 0 logic. This characters think and talk like normal people, sometimes dialogues are even quite complex and about subjects that really makes you think about it.

Overall it's not WOW drama but still interesting, it quite realistically shows human characteristic trough main leads, I like that it hasn't traditional narrative that poor ML is better person than rich playboy-ish SML (for the middle of the drama it's more opposite), also showing that poor people want to be rich/powerful is natural, also these wish can have different shades (ML and SFL for example). At the end all this details make characters real humans not fairytale-ish perfect heroes and I like it. In this drama main 4 characters are all different and quite 3D, though I less like FL and her reasons/characteristics could be better written.

Edit: I don't agree that main couple is bad/uninteresting, opposite - they are very interesting because story is realistic, ML even kind of "bad" is realistic, which is rare... Not every couple who dates has big chemistry and big love from the beginning but in dramas we only see this "true big love stories" especially with main leads, so I liked that in this case it was realistic story/romance. On the other hand we have second couple with cute chemistry and all the vanilla that drama lovers like!
So could be said - this drama managed to show both realistic and fairytale romances, without much primitiveness and cliches.