I'm in awe of Naoki Urasawa's storytelling abilities. He has a marvellous way of handling suspense by controlling the way critical details are revealed, or not. I love his crisp art style.
I think the work as a whole could have been much shorter, with many of the subplots pared away, and overall the series had the feeling of starting out with a brilliant premise but no clear idea of where it was headed. It would have benefited from tighter control.
All this says, this is one of the best manga I have read. Bravo!
Hope you read 21st Century boys as well. Pluto, Billy Bat and Monster are also good. I highly recommend Monster.
I don't even remember the story anymore but I remember getting burnt out by the end of 20th century, and 21th century is even worse. I can't really recommend. Pluto and Monster have the same kind of problem but not as bad.