Are there areas where you've tried to use it and it doesn't work?
Well, one way or the other, you do need to be able to communicate with anothe person. In general, you have two choices for communicating:
(1) "direct" to another person's radio. This is "walkie-talkie" mode, and requires that you be generally line of sight with someone (or at least close to line-of-sight. Your signal isn't going to penetrate a 3 mile thick chuck-o-mountain). Now, the distance can be pretty darned far, especially if you are on a mountain top somewhere.
(2) You communicate to someone else, thru a repeater. Same limitations. Unlike Cell towers, most amateur radio repeaters *are* set up on the tallest peaks you can find, for the best coverage.. That's probably the biggest difference in amateur radio repeaters vs cell towers.
Example: The closest amateur radio repeater to Wrightwood is table mountain. PCT? No problem. Big Pines Highway? No problem. This repeater covers a pretty good part of LA, inland empire, as far north as Kernville. On the southern parts of the ANF, I suspect there would be repeaters on the "LA side" that you'd prefer to use.
Me? Hike? My "hikes" normally consist of a trip between the couch and the refrigerator. Seriously, I've only hiked around the local area... maybe like PCT up toward Heath Wash. No cell phone coverage there, but I've got plenty of repeaters or people I can contact with radio. Someone else will have to tell you how coverage in the Gulch, and those other areas you mentioned.
For me, the radio comes in handy when I'm traveling Big Pines Highway, to get to the city for work. As soon as you get to Mountain High West, kiss cell service good bye, for the trip down Big Pines Highway. There's no pay phones, or call boxes either, so if I have car troubles, or need to call some accident in, radio is the only way.
I believe you'll find an "amateur radio" subject on this forum. We may want to move the discussion there, instead of hijacking this thread too much.
What you might want to do one of these nights is to listen to the "live scanner", using the link at the top of the forum page, any Sunday night at 6:00pm. We hold a weekly net, for the purpose of testing coverages and such. Ok, people generally aren't checking in from Vincent Mine or anything like that, but you'll have a better understanding of the range of the local repeater.