The story is really interesting and really drew me in. Yes, it is a bit naive in places, but at least it is engaging. The characters are also interesting personalities. I'm not a fan of Lee Joon Gi, but for me this was his best role, really played brilliantly. I come back to this drama every year on average, and I usually have the same interesting time with it.
The criteria for rating given on MDL suggest that I give a rating of 9/10, but the criteria not included bring my rating down. FL is woefully stupid. She doesn't understand basic truths. Her selfish "I'm FL so I'm allowed" behavior with which she endangers the safety and lives of other people is outrageously stupid. FL gets 0/10 from me, which lowers the value of the whole series.
I recommend the series. This is the good old school of drama making, where caring about entertaining the viewer without making him a total idiot. Good action scenes - car chases, fights, even shootouts, which may not be very realistic, but are entertaining. It's worth spending time for this drama, as each episode is quite good