How much gravity does their ecosystem really have? I know some people clearly use it, but anecdotally, most people I know bought prime for the shipping.
I keep Prime for Video mostly (kids titles), but We use the free Prime music (not unlimited) quite a bit.
I recently discovered Prime books with kids titles which end up being convenient for car rides or times when carrying many books is inconvenient, but libraries have plenty of assortment for kids ebooks these days so that's not really a selling point.