Of course it has it's flaw, but
20th Century Boys is truly an epic work in several ways. Most of all during this long going story about very remarkable characters. It has some great kind of mystique away from all that rather shounen typical "oh my god, look at that mysterious fighting power".
I guess it's one of the most gigantous main story lines ever. There are barely any pages not telling an actual story, and none of them are irrelevant side stories. Also the perhaps best time skips, I feel like they have been really perfect. Well connected and just at the right moments, without dragging on any part of the story for too long.
Kokou no Hito also has great time skips btw, jumping back and forth over and over again. They feel somewhat natural and not as forced as it sometimes appear in Shounen when -for example- somebody has been defeated and his definings moments are shown again. It's a somewhat different type of skips than in 20th Century Boys though.
In an amazing way it's always clear what timeline happens at which time and what happened in between, without even the slightest bit of explanation and often enough just by showing images.
I think that does a great job in making it all appear very natural, human and real.
This also applies on changing places. These pictures for example show a travel. It was clear that this travel would happen some day, but there was no bridge pictures explaining or anything. Just those of the destination: