I think Fukumoto's art style is pretty refreshing, but it has some interesting idiosyncracies. In a way, it experiences the same-face-different-hair phenomenon. If you looks at both Kaiji and Akagi, it's pretty easy to spot characters with nearly identical appearances. The difference here, I think, is that the repetition is archetypal, i.e. you can pretty much tell what type of man Kazutaka is if you've seen Washizu. The ugliness, the rough lines and sharp angles, really fit the series well. As a series about underground gambling, particularly one in which the organizers are openly sadistic bastards, the rough cut faces are an open manifestation of that. Compare with Higurashi (which some of you have been doing). Higu uses it's moeblob character designs to several ends, all of which apply to the shows overall theme. One, the viewer is compelled to feel affection for the main cast, which ties in to the next one. Two, there is some expectation that the viewer will be disturbed to some degree as the main cast is mutilated or killed. The third reason is pretty much the main one, that being the show's overarching theme that beneath all facades lie a dormant evil. I think this is the reason that Higu was bright and colorufl and moe; it's an attempt to create shock with juxtaposition.
At least, it freaked me out.