A lot of older people these days tend to think it's rude to be looking down at a smartphone during dinner while younger people seem to think it's okay. I believe that whether it's rude or not totally depends on what you're doing on the device.
If you're looking up children's charities or
reading about giraffes then that's not impolite at all, that's good and
it should be encouraged. If you're texting to someone or playing a video
game then that's slightly inappropriate and a little rude since you're
totally ignoring the person you're eating with. But if you're watching
porn or sending photos of your genitals to people, then THAT is rude.
That is where I would draw the line.
No one needs to be listening to
your "2 Girls 1 Cup" music while they're trying to enjoy a nice family
dinner. If you don't believe me, try photographing your nuts the next
time you're at Applebees, well Applebees might not care, but TGIFriday's
probably will. And I already know for a fact it'll get you kicked out
of the Olive Garden so don't even bother.