Coinbase has been the top cryptocurrency exchange for the US market as it’s process makes signing up and buying bitcoin simple; they have seen accounts jump from 5.5 million to more than 13 million in less than a year; their success has not been without growing pains with a number of outages occurring and customers lodging complaints that no one was available to help them; recently they also came to an agreement with the IRS to hand over records of customers involving more than $20,000 which was a compromise from a year long battle. Source.
