I think it's a smart move, Mercedes just didn't want him and I can't see the new Merc car being as good form the off – in fact, I predict it'll be like an '08 BMW season for them to begin with at best, I can see a Mclaren 1-2 in Bahrain easily enough. Ferrari, who knows! Red Bull, for me the real big unknown!
If Mclaren are back on form, Button certainly isn't going to beat Hammy in any other car so he might as well have a go in the same car. And, of all the teams that come and go, there's ultimately 2 cars to be in long term, and his fluke season with Brawn (I think he deserved it but that season was the exception IMO) has bought him a ticket to one of the big two cars to be in, which he might not have ever otherwise achieved.
Very smart move, he might never win again but he has a chance at Mclaren. Also, better to be British in an all British team, playing second fiddle to a fellow champion, than to be British in a soon to be very German team playing second fiddle to a young cocky yet to have been.