As other reviewers have said here, this movie deserves more than a 6+ rating.
As a war movie, it is fantastic in its depiction of the events in one of the shortest invasions in WW2. Good acting and generally good attention to details (equipment, uniforms, tactics, etc.), in addition to a taut, gripping narrative makes it an engaging historical drama.
The strengths of the movie lie in part on its focus on just one small unit and a couple of related engagements over a period of a few hours, instead of trying to tell a wider story that would have distracted and detracted from the experience.
If there is a weakness, it is that there was very little time for the audience to really get to care about the main characters. Perhaps just a few minutes spent on their backgrounds would have been worthwhile.
I would like to highlight the last scene at the end (NO SPOILER). For those who are familiar with the history of WW2, it brings on goosebumps knowing what is about to happen to the Jews in Europe. Fortunately, most of the Jewish population in Denmark were saved, thanks in large part to the Danish resistance and the Swedish government.
If you are looking for a bloody war movie, this is not it. But if you are looking for a brief glimpse into how men react when faced with impossible odds, then April 9th will not disappoint.