In where? O.P. lives in SoCal, but that's a pretty broad locale! On the PCT I had better overall cell reception in SoCal than in most "remote" places that I've hiked.

Of course this is going to vary a lot by the specific hiking trip. Also by what sort of situation you're trying to protect against, i.e., balancing incremental increase in communication against weight carried, cost, and perhaps "fiddle factor" of having to mess with such a thing.

Unless hiking in a virtually urban setting, I don't think that a person can rely on a cell phone for the situation where they're on relatively low ground, alone, and injured. If you just want the option of a hiking partner or a healthy you to hike to high ground and make a call, then a cell phone in some significant chunks of SoCal might not be a bad risk/reward trade-off?