I wonder about the interest.

Interest Only means that you don't actually pay off any of the debt, on ly the interest charged on it.

It can be a bit risky as your equity in the house will not increase overt ime and if the value of your home drops, you could end up owing more than the house is actually worth.  IO mortgages can be a great idea if you need to drop your repayments and free up cash.  Other than that I would only recomend them on investment properties where there are tax advanatages in having interest charged.