A buy down is when you pay additional points to get a lower interest rate. Personally I don't think it is a good investment. Mortgage interest is the only interest write off you have to off set your personal tax liability. Usually you have to pay one point for each 1/4 percentage point in interest. One point equals 1%, so let's say the loan amount is $100,000 -- that's $1,000 to save 1/4% which will calculate to only a few dollars a month. It takes a long time recapture the investment. Because you can deduct the mortgage interest, it would be a better investment to take the $1,000 and put it in a mutual fund or other long-term investment.
Points paid on a mortgage to acquire a primary residence are fully deductable in the year in which you pay them. You have to analyze the costs and benefits for you particular situation. If you intend to stay in the house for a long time, buying down the rate might be a wise move. Points paid on a refinance or home equity loan, as well as a second home (vacation home), are dudectable as well, but generally have to be amortized over the life of the loan. So (theorhetically) if you paid $3000 in points on a 30-year refinance, you would only be allowed to deduct $100 per year. I'm not sure what happens if you refinance again in the future or sell the house in a couple of year. I seem to recall that you can add the unused deduction of the points to the cost basis of the house, thereby reducing any captial gains.
