I'll be in the minority if I have to be. I loved it even with any flaws it had. The chemistry of the main couple is more than enough to make up for the side character/plot shenanigans to me. My husband and I both looked forward to new episodes every week and I just generally enjoyed it much more than I have any drama in the past year or more.
There were some over the top lines to the story, sure, but entertainment value for me was very high. At no time was I bored. I laughed, cried, got angry - just had my emotions all over the place. The music was lovely as well. So take my 2¢ for whatever it's worth. I liked it a lot and I would watch it again.
Kim Jo Won was already one of my favorite Korean actresses, and I'm even more of a fan now. Kim Soo Hyun had the most expression in any show I've seen him in thus far - he really shined in this role.
I wouldn't say there is anything realistic in this storyline but it was entertaining to me and for those reasons I've listed, I'm rating it well above what most people are feeling.