I'd say no one really. It's decentralized but as for it's value that could be determined from a large range of factors: miners and the market. On another point, however, while you technically can do whatever with your Bitcoins, if they're in an exchange or wallet or whatever that's been ceased, hacked, or shut down, your control tapers off as what happened with Mt. Gox and China exchanges right now.
it is decentralized indeed , but how about the 51% attack?
do you think it will still make bitcoin uncontrollable?
i don't think so , at this point it was bitcoin weakness ,
and it could happened anytime , some people can control bitcoin.
I don't you have to worry about 51% attack at all. Mining farms nowadays cost billions of dollars. I don't think someone will invest much more than the best mining farms just to attack bitcoins. It would be much better if he's just going to mine bitcoins as well instead of attacking the network since attacking the network will decrease the price of bitcoins[1]. When bitcoin goes mainstream, there would be investors that will invest even more than the best mining farms of today.
[1] It will decrease the price of bitcoins because the investors will see that it's under attack. Thus, selling their coins.