Art
The art is great. The characters and environments have a consistent and interesting style, and the drawings are a pleasure to look at. Urasawa-sensei does a great job at keeping the physical look of the characters consistent and recognizable at different ages. If there are any complaints to be had, it's that a couple of the characters look rather cartoonish compared to the scenery they're placed against, and that some of the women are hard to tell apart.
Characters
While 20th Century Boys has a large menagerie of characters, they lack any real personality and are mostly defined by their physical characteristics (i.e. Yoshitsune is timid and has small eyes, Maruo is fat, Keroyon looks like a frog, etc). I fear that the creator was too focused on involving the characters with the plot that the audience is never given time to share in the experience of the characters and connect to them on an emotional level. Even the heroine of the story is never placed in a recognizable human situation (i.e. having a crush, being pressured by school or work, dealing with a friend in trouble). She's just a fearless, determined go-getter, and we can only guess at where her unbridled confidence comes from, since we're never shown. Koizumi is perhaps the only character who acts rationally, and she simply serves as comic relief. I must say that having "normal human behavior" be comical is telling of the story.
Plot
The story follows a cyclic pattern of A Revelation / We Save The Day (at least, for now). Since this is a mystery at its core, the Revelations concern clues as to the identity of friend or his next evil plot. The "Who's the culprit? Find out next week!" scenario can be interesting, but it's extended throughout the entire manga and soon becomes tedious. Since there aren't many characters to care about, their actions and speeches more or less pad out the long trip to the closing action of a conflict.
What Conspiracy?
I have a problem with accepting conspiracy theories. It's difficult to believe that so many people in different levels of the plot are "in on" the conspiracy. Even more difficult to explain is the motivations of the conspirators, and moreover, their reasons for not rebelling against the plot or spilling the beans. 20th Century Boys waves a hand over the issue of millions of people blindly following the evil "Friend". There are quite a few plot threads that are left hanging by the end, but ultimately all the audience wants to know the identity and motivations of Friend, and I must say that this is handled brilliantly.
Love, Peace, & Rock'n'Roll
20th Century Boys makes numerous reference to classic American rock bands and world events, such as the lunar landing and Woodstock. Rock reverberates through the story and gives it a unique style. The rock and hippie vibe exuded by 20th Century Boys is perhaps its most endearing trait.
Social Commentary?
It's hard to say whether the bands mentioned are reflective of Urasawa-sensei's own taste, but he does thrust three manga artists into the story who act as his mouthpiece for the hardships faced by manga artists. They exist to showcase "Friend"'s suppression and censoring of the arts, but for me the mangaka were a constant reminder that I was, in fact, reading a manga. I'm still not sure whether creating such blatantly autobiographical characters is appropriate or distracting, but I will note that the ratio of manga artist characters to main characters is about 1:5.
In Closing
I don't recommend 20th Century Boys to the passing manga fan. The concept is very interesting and the events of the story are a fun ride, but ultimately it's stretched out to a tedious length. However, if you want to buckle down and enjoy a manga for a while, 20th Century Boys is worth it.
