Actually, the use of Bitcoin in almost 70 countries is legal and to name a few these are Norway, Poland, Singapore, Slovenia, South Korea, Spain, Switzerland, Sweden, Turkey, United Kingdom, United States, Australia, Belgium, Brazil, Canada, Colombia, Croatia, Czech Republic, Cyprus, Denmark, France, Germany, Hong Kong, Israel, Italy, Japan, Ukraine, New Zealand etc. Being legal in those countries people used it in any type of transactions, e.g. trading, shopping, exchanges, etc. However, for Bitcoin to become the world's leading currency I have no idea.