I saw this film in Cannes 2007 and also went to the party. The boss of Kumagai clan was invited there, who was in the film. My friend took me to talk to him, which normally Japanese never dare to, just because he looked lonely there. He was polite, keeping on saying “Someone like me should not be out in public like this…”
The film is a bit weak as we don’t find how these yakuzas make their living. It must have been the pre-condition between this clan and the director not to touch that territory.
There are funny bits with discovering Yakuza culture here. As a new comer, Naoki has to do domestic work including cooking, serving tea, cleaning and so on for the seniors. His boss is a devout Christian. But I would say this film is an observation of one youth in Tokyo city rather than a study of yakuza world.