I don't know if these are spoilers but these are my general thoughts of the drama.
(I'm currently watching it for the second time. ^^)

Want a smart male lead and his daring female lead?
Want an equally charming second couple?
Want a sisterly bond that transcends beyond limit?
Want a comprehensive plot but not to the point it's too complicated?
Want fun and heartwarming drama?

This drama got it all!

Personally, I like the female character's attitude. She got her fair share of cliché optimism but she was a refreshing soul who had the courage to deal with her own inner conflicts. Also, the male lead is charmingly smart. You won't get bored as he looked like playing chess with the antagonists., thinking a lot of steps ahead to get the advantage. Moreover, I find the second couple lovely. It's like a pairing of a shy white cat and a good puppy. You'll know it when you watch. ;)

And! I must say this! The bond between the sisters is just chef's kiss! It's almost enviable.

As for the villains, you may find them annoying but they were smart and were actually on par with the leads (oh, and vicious too).

PS. I hope one day I watch it again for nth time, the English subtitles are organized and neatly translated.