The biggest problem I have with many of the series based on true events is that they are overly dramaztized to the point that they appear more like hollywood thrillers than a theatrical account of true events.
This is where The Days shines. This series is credible. One can totally imagine that that's how the real situation must have unfolded. Of course, the events have been stitched together to create a narrative that the vieweres can follow but it doesn't betray the authenticity of the events.
We see the characters only within the confines of the Fukushima station but the characters develop nicely even within this limited space and situation. As a viewer, you start to feel their frustrations, fears and trepidations.
The only notable shortcoming that I can point out is that some sequences are unnecessarily long because of which the time urgency of the situation may either be lost on the viewers or may lead to a disconnect for the viewers between time urgency and the actions of the characters.
Overall, I would definitely recommend this series to anyone who is looking for a break from unrealistic, incredible and untrue stories based on true events.