In ideal situations, they're bad. You're basically trading the home you worked so hard to own for income until you die, leaving nothing when you're gone. If you have children or other heirs you'd like to leave something to, this will take your home away from them -- and in most cases you do NOT get as much out of your home as you have equity in it (think about it -- reverse motgage companies wouldn't make the deal if they were going to lose money...).
That said, if you haven't saved enough to live comfortably during your retirement, then a reverse mortgage can work for you. As long as you know what you're getting in to, and that you will lose your home when the mortgage is up (usually when you die). If you need the income, it's certainly better than borrowing it, and in some cases it's better than selling your home outright and moving someplace smaller or more affordable (which is also an option you should consider -- if you have more than a couple hundred thousand in equity in your house, selling it and investing that money in a high-interest paying mutual fund will almost certainly give you more money than a reverse mortgage will).
good luck with your choices.