I'm not a (direct) user of the C++ interop, but everything I've heard about it has been quite positive. A few hopefully relevant resources, first a couple of videos:

- "Introducing a Memory-Safe Successor Language in Large C++ Code Bases" (John McCall; CppNow 2023) https://www.youtube.com/watch?v=lgivCGdmFrw

- "Swift as C++ Successor in FoundationDB" (Konrad Malawski; Strange Loop 2023) https://www.youtube.com/watch?v=ZQc9-seU-5k

And then a series of blog posts by Doug Gregor:

- "Swift for C++ Practitioners" https://www.douggregor.net/posts/

And finally:

- "Mixing Swift and C++" (Swift.org) https://www.swift.org/documentation/cxx-interop/