By most reports I'm hearing lately, it's doing quite well, better than Google Maps in a lot of ways. I don't have an iPhone, so I can't vouch for it myself.
You can certainly pick&choose user comments, but aggregate statistics suggest that apple maps have ~10% of the maps market. Although it's not clear what that number would be if they ever released their maps for Android, it's a good indication that they are a distant second at best. Note that this is a _much easier_ problem to solve than search, and they have not been able to dominate it, 11 years after launch, despite their tight control of their platform.
Slight tangent, but I bet apple maps would have a much greater market share if maps.apple.com actually showed something useful. If I go to maps.google.com, I see maps, which is exactly why it's popular.
Do you have a citation for these numbers? I'm not at all sure how one would go about measuring that. I see some sources from 2020 claiming ~10%, but it's not clear to me where they're getting that information, and it's old info.