Saw Dead from the floor, which is very close to the screen. There was some distortion, but it was still pretty immersive and worked reasonably well for the surround shots/sequences, though I did have to swivel and look around behind me. One of the nice things about a sphere, of course, is that there’s always a fairly undistorted view across from you. The Wikipedia article has a shot from a very low+side seat: https://en.wikipedia.org/wiki/Sphere_%28venue%29#Opening

The sound system + sphere geometry is also interesting for similar reasons, you get good sound no matter where you sit. I even brought ear plugs because concerts are always too loud, but didn’t need them in the Sphere show, they seemed to use their ability to achieve uniform coverage to set the volume at loud enoug but non-ear-destroying levels. (The WP article also talks about the sound system.)

FWIW I saw U2 there in decent but not great seats, and I thought it was amazing. There is a scene where they "deconstruct Las Vegas", so at the end you're just left with a vista of the Nevada desert, and it was so realistic and bright that I felt like I was outside.

Author here: Postcard from Earth only allows people to sit in more central seats, whereas music performances let people sit and stand much closer to the edges. In other words, it’s probably not that bad if you’re in the worst seat, though definitely not as good.