Like Jammer, I use mobile apps more for tracking speed, distance, elevation gain, etc.., than GPS tracking or map reading. I tried several apps, and didn't feel confident in the GPS capabilities of my iPhone. I tried maybe 4-5 apps, and the only one still on my phone after 5 years is MapMyHike. There's a free version and I like it for tracking everything from walks to hikes, but typically for training or day hikes.
Once I get out in the wilderness, though, it's a huge battery and data drain (as most apps that require constant signal are) and the reliability really starts to diminish. Most notably, on a hike in the Northern Sierra (in the Grouse Ridge area) it noted my ~5 mile hike as being close to 30 miles, with a straight line trajectory out and back for ~12 miles right in the middle of the hike. Not something I'd care to rely on if my positioning depended on the app!