First of all, I must proclaim that it is quite a while since I watched this. But: This is one of those shows that you don't forget easily.
I must also admit that the first episodes or so I wasn't really impressed. I think the female protagonist was a bit "over the top" for my liking.
But then, just as a rock that gets polished by the waves, the underlaying themes revealed themselves and I saw the beauty.
I guess my best way of describing it is to take some rather serious themes, spin a story around said themes that is a bit "over the top" to not make it feel too heavy, but rather make you laugh, feel some suspense, and love - both familiar love, and a bit of romantic love – and maybe you even got a bit emotional.

The acting was superb. Seo Ye-ji, Kim Soo-hyun and Oh Jung-se especially all gave convincing portrayals.
Seo Ye-ji's charachter Ko Moon-young (FL) was annoyingly crazy, but also managed to let you see the cracks in her armour every now and then.
Moon Kang-Tae, portrayed by Kim Soo-hyun, made you want to take him in, give him lots of hugs and support.
And then last, but not least: Moon Sang-tae, brilliantly brought to life by Oh Jung-se. He gave so much life and development to his character.

The supporting cast was also good – I can't remember a single one I frowned upon.