The success rate is low, but the number of addresses tried makes up for that. This is an absolutely horrible problem. Microsoft has issued a patch for the problem, so sending a single character of the password is no longer sufficient to connect to Windows file sharing. I have done some scanning for open file shares in the past, and have found lots of systems that have enabled file sharing. File sharing works well in an environment like a small office, in which you want to share data with people close to you and you lack a central server.
File sharing is also common on home networks where there are two or more systems.
Either way, it makes sense to immediately patch any Windows system that uses file sharing, and to use firewalls, even home versions, to block attacks like this one.