It's either great or useless, 100% dependent on your location.

Maybe 40% of my city is missing (top-20 in the US) including absolutely everything in my neighborhood other than a couple parks and churches. It gets my address to the correct block (between X and Y Aves) but that's it. Some small towns around me barely exist, others are absolutely complete.

You'll probably do ok in SF/NYC/LA/Chicago etc, but it gets really spotty beyond that.

Link the area, someone will add something if it comes to this thread :)

OSM is really awesome in (western) Europe. I use it all the time. All new bike trails are in it. Even the trees in down town are in it. It is even great for car navigation. I used the map data to navigate in tiny beach villages in Spain, for a round trip in Iceland and on several other occasions in western Europe. However, i only relied on the data and let my Garmin device create the routes.

I think OSM is a more natural, and even higher quality, match for DDG. I'm pretty surprised by this. Sure, Apple is pretty privacy concious too. At least they want their users to have that image of them. But OSM is on a whole different level due to their open nature rather than a black box with an API in front.

OSM is becoming pretty impressive too. Even smaller towns here in Sweden are decently mapped. A huge difference compared to just five years ago¹, far greater difference than since Apple Maps was launched as far as I can tell. Apple Maps launched in a better state, yes, but not in a good state, and I think it's been moving slowly since then. OSM launched in a minimal state but are moving forward like a maglev train.

¹ Also visualized here: https://labs.mapbox.com/ten-years-openstreetmap/

I'll be fair in that I've never used it for navigation. But I have used it for looking up locations of places, and it never seemed bad. But I don't do that very often.