You want to buy mine? I've got a Garmin 62s that works great for OnX. But I never use it because, as others have said, the iphone app is 10x better.
GPS works fine on the phone. How does it work? Just like a GPS because, get this, it IS a GPS. It doesn't need cell towers. Yes, they use the cell towers to get a faster GPS lock. But for most phones cell towers are absolutely not required. Typically they use the same types of GPS receivers as the better units. The difference is usually the antenna which is crummy. But it's usually good enough. If you're gonna be in deep timber and dark canyons or at high latitudes, yeah, a dedicated unit is better. For most of us, a cellphone GPS is good enough.
The OnX offline maps work fine if you make sure you download them properly. You can test this in the "offline mode." If you see maps, you're good to go. If not, you didn't download them right.
Like I said, the OnX maps and other apps (e.g., Gaia) are easier to use than a Garmin GPS. Battery life isn't a problem. I put my phone in airplane mode and use GPS when I needed it. It lasted about 2-3 days. I bring a solar charger or backup battery and I'm good for a week. Or I charge at night in the truck depending on where I camp. It's a no brainer. If you run out of batteries on your phone, then you're the type that would forget to check or pack spare AAs for the Garmin.
It's one less thing to carry. One less set of spare batteries you need. And the iphone works as a camera, journal, and under the right conditions a two-way voice and text communicator. The downside of a phone is that it really is a single point failure. Not because of batteries. But its not rugged. You drop that thing in water or shatter the screen and you're screwed.
Seriously, I'm thinking about selling my Garmin. If you want it, it is basically brand new and has seen almost no use...because, well...see above.
PS
There are better and free-er maps for the Garmin than OnX. I like GPSfiledepot.com for free maps. They even have a free western land ownership map. It's not as detailed as the OnX but it seems pretty darn accurate. Never seen it differ from OnX. Of course, I wouldn't trust it if I was questionably near private like I would OnX. And a game warden would likely give you the benefit of the doubt if you were using OnX vice a free map from the internet. So, if it was me (and in this case it is), I use the free maps for my Garmin and rely on my OnX on my phone.