I haven't had a need for any of it. I know someone with the doorbell - they say it comes in handy with packages. I'm not sure how as if the package needs a signature, you can't sign for it if you're not there anyway. And if it doesn't require a signature, they are going to leave it on your porch either way. SEems a simple security camera system would be as effective and more useful.
I know someone with the smart thermostat. That seems to be handy for coming home after a long trip and being able to remotely set the temp for your arrival. Walking into a warm house after arriving back home to a cold winter is useful.
I think most of the smart devices may be useful for people with complicated lives. I live pretty simply and find keeping a shopping list isn't a huge task to begin with.
Now when they come out with the smart washer/dryer that will take the clothes from the hamper, wash and dry them, and put them away... Well, then I'll be buying one. Especially if it can remove the bedding from the bed, wash and dry it, and remake the bed. That would be smart!