Many first-time homebuyers that I work with come to me with a budget of how much they can afford on a new home. Often overlooked is the fact that mortgage interest is tax deductible and the cash savings can be pretty amazing. For example, a $500,000 loan has a payment of about $3,000 a month. But when you itemize deductions on your taxes, you'll receive over $600 a month back for the first year! So your actual monthly payment is $3,000 - $600 or $2,400! Don't forget to account for other monthly payments that come with home ownership like utilities, insurance, maintenance and property taxes (which is also tax deductible).
Of course, the tax savings varies on the loan's interest rate and your tax bracket. Yahoo has set up a nice little calculator that gives you an idea of how much money your mortgage deduction will get you.