I really thought this drama is going to be something different and special, and it was... until it wasn't. Others have gone into that, so I won't, but suffice to say, it went from a solid 9 in the first half to completely unwatchable for me in the last few episodes.

The soundtrack became annoyingly repetitive as well.

As for the acting, I think it was great, nothing mind-blowing but nothing bad either. I don't get why some people disliked the female lead, I thought she did great with what she was given to work with.

Both characters had their annoying moments but ultimately it was the need for more and more drama that killed this series for me.
I also found the other characters plot lines quite boring, despite the characters themselves being interesting.

Overall I'd still recommend and praise the first part for its impact and execution, but I advise against wasting your time with the rest unless you don't mind the shortcomings.