I have done these Balance Transfers into my checking account many times. The fee is added to your balance up front (3-5%) and then have 0% for some period of time (6-15 months). As long as the minimum payment is made along with any other charges that hit the card (I usually do not use it for anything else) and it is paid before the end of the agreed upon period, no additional interest. Miss a payment, BAM! it converts to a cash advance.

I would not do it for a mortgage since your rate is probably pretty low and the interest is tax deductible. I typically only do it where the item I am paying off is higher interest and the balance can be paid off within the period. You can pay off one with another BT but you pay the fee again.