I would just like to specify that this is not a bad drama. All in all I enjoyed it, but it is definitely not without its flaws.
 
The story: The first couple of episodes were very good, but sadly, that did not continue for the rest of the series. It became a one story per episode kind of series that felt very predictable. Though they tried to keep the main plot present throughout, it didn't really feel like one story told over several episodes, but more like a story referenced in some of the episodes. The side-characters were by far the most interesting and were a crucial part of my enjoyment of this drama. The best episodes are the first couple and the last couple of episodes. As the last episodes rolled around I came to remember why I liked this drama so much in the beginning. The story is not a bad one, but it had the potential to be much better.

The acting: If you have ever seen any drama with Lee Jong Suk you probably know what to expect. He seems to pick the same type of character to play in most dramas he appears in. He acts quite well, but I didn't really feel anything from his performance (except for a few scenes in the last episodes). Suzy was probably the weakest link in this drama. I wouldn't go so far as to say that her acting was bad, but she won't be winning any awards either. She mostly has the same face throughout the entire drama. Neither did she have any chemistry with Lee Jong Suk. I didn't really feel anything from their relationship. I neither loved it nor hated it. I would not care whether they stayed together or broke up. The rest of the cast did quite a good job. As I mentioned before, the side-characters were done very well. The child-versions of the main characters  did a good job as well.

The music: The music was quite good. Not much more to say about that.

All in all this is an enjoyable drama. It does feel kind of repetitive so I might have liked it a bit less if i saw it all in one go rather than watching it as it came out. The acting was ok, but lacked in some ways. If you dont mind a one-story-per-episode kind of show I would recommend this.