You might notice similarities in the presentation and some of the themes if you're familiar with series such as Ashita no Joe, Attack No 1 and Ace wo Nerae, however Kyojin no Hoshi is more about family, and the individual relationships of the MC than those other series, these relationships are explored through fierce rivalry, drama and struggle that is constant throughout the series. The struggle explored in the series is both physical and emotional.
The art in the series is incredible, utilising visual gimmicks that you might be familiar with from series like Attack No 1, the animation and the art also enhances the culmination points of the series. Visually the art gets more detailed on those parts which makes you feel tension of the situation through that visual presentation. I'd say the usage of such presentation enables another level of engagement from the audience in those parts where the tension and struggle culminates. The series also features lot of sports commentary which I think makes the experience of the matches feel more immersive as it is almost as if you were watching a real match.
Other than the above visual presentation I mentioned the series also puts emphasis on the emotional aspect of the sport and portrays it visually creating impactful visuals throughout the series. While the method that is used exploring some of the themes is more focused on the relationships of the group and family than the individuals own struggle like in Ashita no Joe and the others I mentioned earlier, I'd say that it doesn't really make the any less enjoyable. The struggle when it comes to the relationships in the series is also presented from multiple perspectives, and how different characters experience their relationship and struggle relating to that in different ways.
There is also a sentiment about war and tragedy explored in the series as one of the themes, featuring footage from the war and exploring the effects of war to the individual, relationships and to the sport.
Throughout the series as you grow with the characters you start feeling the weight of their struggle yourself and get immersed to the deep relationships and dynamic that the characters share, I'd say this enables another layer of enjoyment when it comes to the series.
Overall I'd say that the series is an epic journey through youth and the struggle relating to that and what it means to when it comes to the relationships as you strive towards something as specific as success/sport and presenting a question if it is worth that price.
