While most people claimed they lost interest in the second half of the series, I found the first half very plain and the second half better – which led me to give it an 8/10 overall when it could've been as low as a 6/10 up until episode 7.
Now, don't get me wrong. Each episode from 1 to 7 wasn't bad; they just didn't add up into a good 7-hour experience for me. They were good one-by-one. If you have seen American series (police or crime series in particular), you'll know how they start an episode with one case and ends it within that episode, and nothing from that episode will affect the next episode at all. As soon as our main detective Park Kwang Ho arrives in 2016, Tunnel plays that card.
For character introduction purposes, not touching on the main plot for a short while is totally okay. However, the length of 7 episodes of doing that is painfully long – 44% in and there's still no progress whatsoever. That point kept bugging me all the way. Apart from that, the main characters are good at their job, so the one-case-one-episode is averagely fun on its own.
I believe that the story picks up around episode 8. From then on until episode 12, the pace of the drama suddenly changes in a good way. These are the most exciting and suspenseful 4 episodes in my opinion. There is a surprising amount of improvements from character development to story revelations. The last phase of the drama, episode 13-16, cools down a lot, but it still keeps playing with the main story. For me, it was fine. By the end of the series, I was more satisfied than I thought when I went through the first half.
It's still too bad that after all those satisfying second half, I still don't see the point of making this a time-traveling drama. Of course, the "tunnel" and the time-traveling play a part in this story. I just think that they could've made use of this theme much more – or dropped that idea completely.
The main character who time-travels lacks a lot of "oldies" qualities I expected to see from its premise. By this I mean that he could've possessed old-school yet surprisingly useful methods that help with the modern crime-solving process, or rather appear really countrified that he looks funny. Instead, he adapts to the modern world so well. If this wasn't a time-travel drama and this character was a detective from the present time, it'll still work just fine. It feels almost as if the writers are using this "time-traveling" trigger word just to call for attention.
At the same time, Tunnel cannot escape a lot of clichés in making their characters and criminals, and even situations. A lot of things were revealed by words rather than actions. Many mysteries are solved in a blink of an eye after dragging them for so long. It's like when things build up to a certain degree and you're almost excited, the next moment it becomes "Oh... That's it?"
The cast and their acting really save the entire show. They're extremely great in every given situation, although again, you'll have to get to the second half of the show to see so. I rated their acting a 10/10 although some small side characters are so-so because they don't have much impact anyway.
The background music and sound effect cues aren't awkward, so I gave it a 7/10, but they mostly just went through my ears instead lingering around for more impact. This is the genre that doesn't need emotional OSTs to play at heartbreaking moments anyway. I still wished I ended it with one or two songs to keep listening to. I ended up with none.
Final thoughts: I wouldn't suggest my friends to watch this for the time-traveling. I would suggest them to watch simply because it's a good crime drama with good acting. I liked it – don't get me wrong after I point out all those cons. I didn't regret watching this show. Also, I barely rewatch dramas, and a crime drama can be either too predictable or too gruesome to rewatch, so I gave that part a low score.
Overall, it's a 8/10 for me because it does only 75~80% of what it initially promised to do. This score is not to be compared with other dramas, especially the ones that are not from this genre.