Everyone seems to be saying this was a bad move by Jenson, but I disagree. It seemed to me that Brawn/Mercedes wanted to get rid of Jenson, there was no reason why they couldn't at least meet him in the middle on his wage demands. I suspect they wanted an all German driver line-up, and if the only reason they would have kept Jenson was to save money on wages. If Ross Brawn valued him so much then why not give him more money? It's not like the demands were unreasonable.
Jenson might find it hard to adapt to life at Mclaren, but I think he knows his position and won't be expecting to beat Hamilton to the championship every season, unlike Alonso. Also, the Mclaren team has changed since they had Alonso, Ron Dennis stepping down and other issues seems to have made them a bit more relaxed and less clinical. It will obviously help that he is a British driver in a British team as well.
So I think it's a good move for everyone, Jenson can settle into a long career as Mclarens 2nd driver and Rosberg can be beaten by Nick Heidfeld on a regular basis and prove that he is far from the best German on the grid.