I have an 80/10/10 loan on my current home.
We priced conventional w/pmi, VA (because DH is military), and this loan. It was considerably less expensive in the long haul to go this way.
Think about it this way.
With a PMI payment, you're paying a premium to insure your loan on top of the house payment for 95% of the value of your house...for the lifetime of the loan.
With an 80/15/5, you're strictly paying for your mortgage...it's just 80% to one person, 15% to another.
Typically, your 15% is done on a shorter term, so your payment may be as high as it would have with PMI, but that 15% mortgage is usually amortized for a shorter period of time, such as 10 years.
After that 10 years, if you're comfortable with your mortgage payment, you can use the payment you had from the 15% loan to apply to the principal of your loan.