As long as Vettel shouts 'OMG ASDFSDAFAF THANK YOU BOYS' and sticks his finger up every time he wins a race or goes on pole, and sticks his bottom lip out and whines like a child every time he doesn't get his own way, I'll never take him seriously.
To me, he comes across as young & boyish for sure but in a nice & enthusiastic way. I don't see any malice in him and I don't think he's particularly sulky either (Hamilton surely takes the biscuit on that score, with Raikonnen not far behind
In terms of overall driver ability, I'd definitely put both Hamilton and Alonso ahead of him, but Vettel is one of those guys with incredible speed who's unbeatable when everything hooks up right for him. He's fantastic in qualifying and when out in front.
I don't think he's anywhere as good a racer as Hamilton, he's not as mentally strong as Alonso or Schumacher (in his prime), not as smooth as Button, and not as persistent as Raikonnen. But he's a very fast driver who, when given a great car, uses that car to dominate races in a way a lot of other drivers can't. Should he have 3 titles? I never think of sport as a "should" game; I prefer to let history sort out that side of things. Each year's championship can stand on its own merits.
On another note, I see Webber has called Grosjean "a nutcase" for crashing antics. The BBC have put together a highlight (lowlight?) reel of all Grosjean's first lap crashes; it's pretty funny. At least, until you see the one where Alonso almost got decapitated. I'm far from convinced Grosjean should keep his F1 licence...