I'm one of those that think the back feels like a textured plastic, because it is a textured plastic. Doesn't feel close to stone or ceramic at all - it doesn't use those materials, and doesn't feel like that. I definitely don't think it feels premium. I don't know why they bothered with the aluminium metal build, when they could have just made it all of plastic, saved a bit of coin and it'd feel the same.
The phone feels hollow to me, but I think that's because of the cheaper vibration motor they're using. Previous Pixel's had quite a nice vibration motor when when I'd type, it'd get a really nice 'sharp' vibration when typing. Now it feels more loose.
The more I use the phone, the less I like it to be honest. The only thing it improves on from previous Pixel phones is the better battery life.