If you've already maxed out your contribution to 401(k)s and IRAs - both of which provide tax deferral without annuities' extra layer of fees - then variables can be a way to boost your savings for retirement, although you'll probably first want to check out investments such as index funds and tax-managed funds, which can provide tax-advantaged returns at a lower cost.
If you do decide to buy a variable annuity, you should limit yourself to one with low annual fees and forgo expensive options that dampen long-term growth potential.
If you want more of a potential payout than a fixed annuity offers and are hoping to benefit from market returns and have control over your investments, you may want to consider a variable annuity.