I think they’re great - absolutely perfect for situations with young kids who are constantly moving, or just to capture little vignettes of movement that bring a photo to life for a moment.
But support for them is virtually non-existent outside the iOS Photos app.
You can extract them as a pair of HEIC and MOV files, but I know of no Free photo management software that is capable of associating them and displaying them together (and yes, there are internal IDs in both files that make it possible to link them, so they aren’t quite as fragile as most sidecar files, although they are still fragile because Apple for some insane reason decided not to make use of the HEIF container format’s native ability to store both in the same file).
I’m unsure if Android has an equivalent feature or how it’s implemented, but I’m pretty sure no DSLRs have anything like it.