I think they are great, except when people talk or text while driving.
During Christmas shopping, I could be checking prices in one store while my daughter was checking in another, saving us hours of time. Also, they can be a lifeline if you are stranded somewhere.
Granted, cell phones can also be a stone around your neck. I simply turn mine off in restaurants or other places where I don't care to talk, because you can always just check messages when you are ready to.