The website says that it is both free ("We don't charge fees when you convert funds to or from bitcoin with a linked bank account, when you store your bitcoin, or for bitcoin transactions.") AND insured ("All of the money in your Circle account is insured at no cost to you."). I am not sure how that will work out for them; given the short and eventful history of BC, I would assume that insurance charges would be quite steep.

But then: I seem to recall many money changers at airports also advertising "zero conversion charges", and they seem to be doing just fine even while paying (what must be) high rents for their spots. So perhaps there is a revenue model there which I can't suss out.