Johan is... an odd villain, if I say so myself. He's by no means ineffective; whenever he appears, I am just SHOUTING to the characters "RUN AWAY! He's gonna kill you! Or worse!" and he is very diabolical. Of course, there are times when I think he's a bit out of place in the manga. His ability to figure out the nature and intentions of any person he briefly gazes upon and instantly find a way to turn them against the person is taken so far that it's better for my sanity that I assume he's psychic and run with it. Also, he seems to have this ability to actually exude an aura of fear and dread in people like a Bleach Shinigami. As this is used to set him up as a frightening monster, I feel this is cheating, not to mention he doesn't exactly live up to it when most of the bad things he does only actually happen because the people around him stop his incredibly elaborate schemes from failing (I'm actually more scared of Roberto than Johan, for instance).
And then there are his intentions. Anyone who's seen the ending will inevitably be royally cheesed (I was, no doubt about that) and will know what I'm talking about. Still, I get the impression that Johan doesn't do what he does for kicks. I'm not saying what he does is for necessities, but I get the impression that he's trying to learn something. He's obviously trying to enforce into Tenma that the world sucks, but I think it's the "seed" of things he's looking for. What makes a human? What makes a monster? With all these questions I imagine running through his head, his young age of 19 is surprisingly fitting.
Actually, he's 20 - 22 over the two years that most of the series (after the time skip) covers. Still really young, though.
He is an odd villain. He has mentioned doing extensive research though (apparently questioned possible witnesses before verbally destroying Richard Braun) and has a large network of people willing to do his bidding. I chalk up his knowledge to doing his homework and/or assigning it to others. He's also good at reading people and guessing what they'll do, though he isn't 100% accurate.
His ability to exude dread does seem overblown... The series spends a lot of time trying to make the watcher guess whether he was the literal antichrist or not. Jury's still out on that one.
I will say I think he does live up to all the hype. I wish characters wouldn't talk about/hype him so much, because it starts feeling cheesy... but he backs it up. Convinces the children in a city to knowingly play death games on roofs, talks or nearly talks a couple of people to death, persuades serial killers to be his fanboys/lackeys, actually creates at least one serial killer (the taxi driver), hides in plain sight, and shoots several people himself. His aim is good. >_o
I wasn't cheesed, actually. Confused for a while, sure. But one of the things that is interesting about Johan is how he switches his end goals a couple of times during the series. He admits that, yes, he was going to do something, but now he's seen/remembered something bigger/darker still and knows the "true" ending.
Sometimes I wonder if he /had/ an ultimate goal before he settled on the one in the finale. Being the last person on the earth seems like an evil child's goal. Once he grew up, he may have settled for creating as much true fear as he could.