Most of Dai-Nipponjin is filmed in documentary/interview style, telling the story of Masaru Daisatou. At first he appears to be a down on his luck every man. He is divorced, living alone with a cat and for some reason everyone seems to dislike him. It is not clear why anyone would be interested in showing his story.
However we soon discover that Daisatou is Dai-Nipponjin, a giant super hero that battles the giant "Baddies" that periodically attack Japan. Most of the time Daisatou lives his life as an normal person, but when the Baddies attack, he is transformed into this giant hero. This has been his family's job for 6 generations. His grandfather was viewed as a hero and treated equally well, but no longer. Daisatou is seen as a nuisance, his battles are loud and cause much destruction, generally annoying the population of Japan.
The movie mostly follows Daisatou around, showing him in his daily life. Dia-Nipponjin tries to show what real life would be like for Daisatou, interspersed with this super hero activities. The comedy is mostly based on the ridiculous nature of being a giant super hero and the conflicts this causes with day to day life.
I generally like this film, however sometimes the documentary sections seemed slow. If you are familiar with Japanese giant monster fiction (more Mighty Morphing Power Rangers than Godzilla) you'll probably find this movie crazy enough to enjoy, (especially the ending). However if you're not, you might not get the references to really enjoy the movie (especially the ending). All in all, if you read the above description and said to yourself, that sounds awesome, then go see this movie. If it made you think, why would I want to see that movie, you probably don't.