I would, of course, never recommend an ARM to anyone who is financially undisciplined. On the other hand, a 40-yr, fixed rate, interest-only loan could be just what the doctor ordered in terms of flexibility for an experienced investor who wisely makes escalating extra principal payments to pay off the loan early.
I realize there are alternate strategies, but, for me, it's all about the cash flow, so the loan product that offers the lowest monthly payment, with no prepayment penalty, in the long run always seems to fit best into my overall plan.