If you set up a separate account, you probably don't have to worry about hitting the 15G limit unless you have many, many photos or are taking them at very, very high resolution.
We pay for family subscriptions to Google Play Music and Spotify. My wife and kids listen to music on our Google Home device and they do a lot of Youtube. I didn't want to do Spotify, too, but the older two asked for Spotify for Christmas because of the playlists only available there. $14.99/month x 2.
I was a devoted Google Reader user and I don't trust Google very much to keep things around. But it's easy to share the links with family and let them download the photos to their own storage.
I was also using Amazon storage to backup when they stopped doing the free backup storage, cutting it to 5GB (I think) for free. So I don't trust them, either.
I hear video is pretty popular these days.
What really killed it for me though is that the Amazon Drive client software is terrible. Slow and extremely buggy: it appears to do file operations on the UI thread, so the whole UI freezes for minutes at a time.
But one of important benefit I use Prime for is Amazon Photos. Unlimited backup of your photos. When I look at other solutions such as Backblaze, they will all cost at least $50 a year.
Then another benefit I like is Prime Music. Which again is going to cost money if I get music subscription from Spotify or somewhere else. So when I add cost of these two benefits on their own, Prime becomes pretty good deal.
What's your opinion on these benefits?