At present, bitcoin is not valid in all countries. But I believe in the near future Bitcoin will be the common currency of the whole world. It can replace the existing internet payment systems, instead of paying by credit card over the internet
Really bitcoin cannot replace Fiat money, but he can replace credit cards. Because it is the same method of doing payment in the internet. And credit card is controlled by governments. It is not good for all of us.
Yes, bitcoin is best used in terms of digital used. It can be global currency when in digital, but cannot replace fiat currency. They do have best used, so i suggest that used both for your satisfaction. But we really don't know what will be the future of bitcoin, maybe our speculations are right or maybe wrong. So the only thing we can do is to wait for it to happen.