They are covered by a lifetime replacement guarantee (simple registration required, no spam). They're well worth buying.
Drop them in your purse; put them in your pocket; carry them in your wallet (they have a hard case); carry them on your keychain (they have a keychain case available); stick the case to your cell phone, or to your computer monitor, or to just about anywhere. They're very convenient, especially if you don't have regular reading glasses handy.
Give them a try. They have a 30-day money-back guarantee. They come in various colors, and strengths. What do you have to lose?