"What are the things about SFI that most convince people to join?"
Good question. Track record of SFI itself explain the genuine nature of it. If a business is not genuine than it can not survive for such a long time. If you see the quantity of members joining daily worldwide that also explain the reality of the business. There are members who joined SFI in the year 1999 and still active. If SFI is fraud than no one would be a active member for such a long time.