I have an iPad3 with 64GB, which has 44GB free. My iPhone 6+ with 64GB has 39GB free. My mid-2012 rMBP with 256GB SSD has 138GB free. My 2015 Yoga with 256GB SSD has 184GB free. My 10.5 IPP will have 64GB, if it ever gets here.
System usage is different for everyone. But I don't understand why people say they need 512GB on an iPad because they watch so many videos. How often do they watch them? If they watch enough to really justify that much storage, they must spend all their waking moments watching videos.
I don't treat my storage like cache for videos and music. Maybe it's just me, but once I watch a movie, I'm done with it, and I delete it. I have no reason to keep hundreds of movies on my system. If I want music, I listen to Pandora or Spotify. The days of ripping CDs onto your phone so you can carry all those tunes around are over.
And then there's the cloud. If you want to maintain a mammoth library of videos you watch once every blue moon, put them in the cloud, and then you get at them from any device. Why load up your mobile device with them?
I just don't get the need for so much storage on a mobile device. That said, if the new iPad offered 128 GB, I would have gotten that. But 256 GB? That's crazy. For me, anyway. YMMV.