FRS-band radios are advertised as having a two-mile range. Slightly more powerful GMRS (General Mobile Radio Service) units have a range of up to five miles. But, for GMRS radios you need an FCC license. Some portable radios have both FRS and GMRS bands, though. An example: Motorola's Talkabout T5720 ($50 per pair; www.rei.com).
Anyway, that's all background chatter. Do they work on a trail? Within certain pretty tight limits, yes, but remember the two-mile range pertains to ideal conditions. Remember, too, that radios such as these are line-of-sight devices. If one radio can't "see" the othermeaning there's a mountain or rock ridge in the waythen they can't communicate. Dense stands of trees or large bodies of water may also interfere with radio signals. When I was active in mountain rescue, we had fits in the woods even with our very powerful military-style two-way radios, which under perfect conditions had ranges of many, many miles.
So, my sense is that the working range for these radios on the trail is somewhere between a quarter and half mile, and very rarely a mile or more. And, it's certainly less if one party turns a corner around a hillside.
Still, for $50 you might well get some use out of these things. I'd say they're great if, for instance, Party A has left camp to go fishing at a nearby lake, and Party B wants to call Party A back to camp for supper. Or, if Party A is hiking up a nearby hill that can be seen from camp, and wants to radio their progress.
I'm very reluctant to recommend these things as "safety" devices, as in, "If you get hurt or lost, just radio us." Because the odds are very high that when you most need these to work, they won't.