Generally, I think they're fine. I've had good success with them on ski slopes, where they can help you keep track of a party. But that's often pretty open terrain. For climbing, they might have some utility over short distances when communicating about routes, rappelling, belays, that sort of thing. But the odds are good that the minute you're out of sight of your partner, you'll be out of radio contact too. I know from my years of mountain rescue work, using very powerful, high-end two-way radios, that communications in rugged terrain is extremely unreliable under the best of conditions.
Points to look for? Price, really. More money doesn't buy you more range. And be wary of radios that advertise "GMRS" (General Mobile Radio Service) frequencies in addition to FRS frequencies, as you will need to apply for a license and pay a fee to use those frequencies.
In general, my view is that FRS radios are useful convenience gadgets under benign conditions. I would NEVER come to rely on one in what might be a critical or life-threatening situation.
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