LOVED THIS DRAMA SO MUCH!
The storyline was so well written, you hardly find gems like Signal in the drama world. Everything happened for a reason, and even though it seemed like they were dragging it a bit towards the end, the final result was really good. It was such an enjoyable and interesting experience, it took me a little bit to be completely invested into the characters and the plot, but once that happened there was no turning back.
I kinda binge watched this drama, something I hardly do with any tv serie, so that's already significative lol.
I have to mention that the best thing of Signal was the performance of the actors: everyone did so well, they all handled their characters with such maturity and carefulness, you really don't see such mature and experienced actors anymore.
The music was nice and with a vintage touch, really appreciated it.

So, if you like thriller, crime, corruption, serial killers and dramatic suspense, with bare minimum and well-handled romance, this is definitely the drama you were looking for. If you still don't like all that, you should still give it a try because there really isn't anything bad about it!
imo SIGNAL IS A MUST WATCH!!!