Most people have been panning him for years in F1 (certainly the armchair racers have), even after showing an amazing amount of loyalty, bringing home both championships, and showing some incredible talent, he *still* gets panned by the futon fanboys and some of the industry pundits.
Some people are still flogging the dead horse of money - It's absolutely *not* about money - he's actually on less money than if he had re-signed and continued to race for Brawn/Mercedes.
It's fairly clear from what he has said in numerous interviews already that he wants to prove his critics and doubters wrong by taking on Lewis in his own garage - He knows it's going to be the challenge of a lifetime, and that's exactly why he's doing it.
Jenson knows how to fight, how to bring home the car and the points, and knows that McLaren are one of the best development teams on the grid, it's not going to be easy, but he's under no illusion that it will be.
He also has a 3 year contract, which appears to be getting rather scarce in F1 these days, and that Mercedes were only offering a 1 year dear with a 2nd year option shows who valued him for the long run more.
Good on him I say - at the very least he'll be another DC for them, but with the added bonus that he has already won the ultimate prize already. But he has the option with hard work and determination to indelibly leave his mark and stand up there with the other great names in the history of the sport
It was the only real sensible career move - he could well have been left high and dry in Mercedes in a year or so without the option to jump to another competitive team - he now has a minimum of another three years at the sharp end.