Endings

I agree, ambiguous conclusions don't work for every story. It's another reason I have so much respect for Urasawa's writing - he manages to pack so much detail and suspense into his stories and yet still resolve all the conflicts by the end. In Monster's, I feel like he left us with an open-ended question for each of us to answer individually. There were so many richly developed characters with so many of their own motivations, strengths, and weaknesses - so it makes sense that each reader would have a different opinion of who deserved what and whether Tenma's actions were, in the end, justified.

20th Century Boys and PLUTO were also equally compelling, I'd have to say. ;>

I have read both of those too. He is who I regard to be the best in mature manga writing as the themes he introduces are issues we're struggling with at present and also the future (Think sentient robots, as in Pluto) and the way he writes and draws the scenes reads more like a good novel than simply a manga. His characters also have interesting facets that both outline their roles and yet also makes you think twice before pigeon-holing them which is difficult to achieve to say the least.