Wow, nothing short of a masterpiece IMO. Brilliantly filmed, edited, scored, acted, scripted and directed... in fact I don't think I could fault the film in any aspect. Be warned that it is violent though. Very violent. Stylish, surreal and very very violent - one of the most violent films I've ever seen in fact. Not HK-style millions of bullets and a huge body count (counting survivors in GONIN is more appropriate), but a much more japanese style of brutal, realistic violence. Takeshi Kitano films are obviously a good point of reference, though GONIN isn't a Kitano film at all, despite his prominence on the DVD case. His part is actually very small (but classic Kitano).
The plot involves a guy who runs a disco, who gets into financial problems, so he borrows money from the Yakuza that he then can't pay back. He comes up with a plan to rob the Yakuza of their own money, and assembles a "motley crew" to carry out the plan. This annoys the Yakuza. You don't want to annoy the Yakuza.
The style of the film is somewhat reminiscent of the Korean film Nowhere to Hide, but more violent. Did I mention that it was really violent yet? I'd probably take bets on the director of NTH having seen GONIN before making his film in fact.
I think that's all you need to know. If you're a fan of Takeshi Kitano, extreme Japanese cinema and very well crafted films, get GONIN in your next order.