I think McLaren regret it more than Hamilton, they have taken a risk on another young driver who basically hasn't produced anything since they signed him whilst Hamilton has cemented himself as one of the very best. There is a rumour that Ron Dennis fecked up in the negotiations by taking the attitude of Hamilton needs McLaren more than McLaren needs him.......but its just rumours, maybe bullshit but something definately went on between Dennis, Hamilton and his management team.
Nothing wrong with trying something new, I still think being in a high budget manufacturer team might pay off come 2014, if Merc don't seriously improve next seasons car it might be a season to forget but he had one of those in 2009 anyway and still nabbed a couple of wins. The driver market will be wide open by the time Hamilton is back on the market anyway and he will be only 31, he probably has another 10 years minimum in F1.