In practice I've had overwhelming feedback praising our use of spinners in the Heroku CLI and not a single complaint I can think of. In fact, I've had more praise for adding spinners and color than any other change we've put in over 4+ years of development.

That said, you need to be careful. Don't use a spinner if it's not a tty or TERM=dumb. Do use it in some CI environments that support it (Travis, CircleCI) That handles all the issues we've seen and everyone seems to be happy.