Yes, this drama moves slowly at times, so it depends on the kind of drama you like to watch. If you're someone who loves the slice-of-life type show that lingers on moments of reality, then this could be your type of show.
I started this show because the director is behind two of my favourite dramas of all time, Misaeng and Signal. I loved those dramas because of the realistic way in which they depicted human relationships, and My Mister is no exception.
This drama features a lot of stories around loss, as well as the aftermath and how different characters cope with loss. All of the characters felt relatable in the ways that they dealt with loss. As I mentioned earlier, this isn't one of those dramas filled with thrilling moments of action. Certainly the corporate drama has some tense moments, but at no point did the drama fall into the trap of using overdramatic cliches to drive the story along, despite having some familiar elements (the ones you may expect in a typical corporate drama, where characters are vying for higher positions). The ending was appropriate and realistic.
In terms of the main characters, I have to commend Lee Sun Gyun and IU for playing their roles well. Their relationship is likely my favourite platonic relationship in all of drama land, in that it really highlights how each character grows with the relationship. It is certainly not one without imperfections and power imbalances, but I was never disappointed in the direction the drama went with these characters.
The family and friends are a really lovable crew when it comes to supporting one another, and they make the drama a heartwarming one to watch. Like any family relationship, the brothers have their ups and downs, and they have their likeable and unlikeable moments as well. By the end of the drama, I was left with an appreciation for the different character dynamics and how they came to be who they are now in the current timeframe, in which we get a glimpse into the lives of their small community. My only complaint is that the workers at the company are more two-dimensional. (But then again, if you're looking for a really good drama focused on company workers, there's Misaeng.)
Music choice was also well done.
Overall, I would strongly recommend this drama if you're a fan of this director's previous works and are looking for a drama that really does a good job in depicting the nature of human relationships. It was at times very serious, but also heartwarming.
If you're looking for something lighthearted, a romance drama, or something filled with exciting action, I would give this one a pass.