Before you go hog wild over the tax benefits of the mortage interest deduction, remember that you give up the standard deduction which is substantial. If you have a lot of other itemizable things then it may make sense but 9 times out of 10 people overestimate how much the interest deduction is saving them, because they don't realize that they "lost" the benefit from the standard deduction.
Example: if your standard deduction is $5k and your interest payments are $9k, you only get the benefit from $10-9=4 times your marginal tax rate (at 25%*$4 it would only be $1k of incremental savings). The mistake people make is they multiple the $9k by 25% and say "man I'm saving $2250!" Assuming that they don't have any other itemizable deductions, of course.