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.
OK, sure it doesn't actually feel like stone or ceramic but that is in part because it's not cold and dense. Purely from a texture standpoint, it does feel like matte ceramic in my very humble opinion. The comparisons to cardboard paper are harsh and pointless for the 80%+ of people who wear cases on their phones. I agree, the phone does feel hollow...I noticed that if I tap it with my fingernail it has a hollow sound, whereas my S20 felt dense. If the Pixel 5 were more dense, I think it would feel a little more premium, but I just don't think density alone and out of context should be a gauge of build quality or premium feel. Also, it may be covered in plastic, but the frame is aluminum and you can tell. You can also tell it's a little more hollow where the center was cutout for the wireless charger. This does take away from the hand feel.
If you're comparing this to the Pixel 4...yes the vibration motor isn't as good and it doesn't have a "premium" glass build. Compared to my old S20 it's noticably lacking the in-screen fingerprint reader, 120hz, a brighter screen, smaller selfie camera cutout, and the telefoto camera. If you want cutting edge hardware-wise, Galaxy or OnePlus is the way to go. For everyone else I would recommend the Pixel 5.
Honestly,
this phone reminds me of the old Nexus 5. It's not quite bare bones, but it's just the essentials of what a phone geek wants/needs at a semi-reasonable price.