If you want to assure that the money will flow regardless of how long you live, an immediate annuity can be a good supplement to your retirement portfolio to help cover basic living expenses.
The greater you feel your chances are of living to a ripe old age, and the more worried you are about outliving your money, the more of your assets you might want to devote to an immediate annuity, although you'll certainly want to leave yourself enough assets outside the annuity to meet unexpected expenses, to fund the occasional splurge or to provide for your heirs. 
