The nothing for heirs is the largest downside of annuities; the large sales commissions and expenses are another, but they do provide a cheaper means of guaranteed income. The other thing to pay attention to is the financial security/soundness of the annuity company. $275K for $20K/yr is a little over 7.25% per year. That seems achievable while consuming principal, but if the annuity company goes bankrupt then you're out of luck...
You also need to consider social security. If you only need $20K in annual expenses, social security is likely to cover well over half of that.
For us, we've got heirs, so we're going the self-funding route rather than use annuities.