We did a 5/5 arm with an 80/15/5 and avoided PMI through the 15% being in a HELOC. They are hard to find but they exist. We did it this way because the property we wanted had potential for growth, so we kept the extra 15% of our 20% down payment to finish the basement and then refinance.
We finished the basement after 3 years and refinanced before the arm had a chance to go up. Had enough equity in those 3 years after adding the large basement to refinance under 80% LTV and got a killer rate.
So in my opinion arms traditionally suck, but if you have a plan and can stick with it they can really save a lot of money and allow more freedom with your money.