I find the episode 1 quite good, nice story background, character background, Shin Ha-Kyun acting, etc, rate it 8/10
Then in episode 3 and 4, quite draggy, it could be compressed into one single episode.
The best episodes are episode 7 and 12, I would give those episodes a solid 9/10

The rest episodes quite weak and draggy. Sometimes they just confronting each other while the evidence still not enough which becomes repetitive. And then the kept secret for later episodes, makes the earlier episode quite draggy because they can't talk about it yet. So the story building things not quite good.

And the last episode is quite nice closure, not great, but still good, I give it 7.5/10
Shin Ha-Kyun's acting the most shining one. On those episode 7 and 12, there are also some great scene throughout the series.

If only they can maintain the interesting things on each episode, I would have rate it higher.
For overall series, I would give this 7/10