Re: Who controls the Bitcoins?

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

You seem to be confusing billions with millions

The whole market cap of Bitcoin is below 20B dollars at present, so there is no way some miner (or even all miners combined) could have invested that much in the mining equipment. Apart from that, there may still be a viable reason to perform (or make an attempt at performing) a 51% attack. If miners have already earned decent profits, they may collude to try this attack (basically, pretend to attack) to bring Bitcoin prices down to buy up cheap bitcoins (again). Besides, this is what we could well expect next from the Chinese authorities in the their ruthless fight with Bitcoin in China (and a lot more)