Yes, you get playbills. And if your concern is seeing Bradley Cooper in the show, I think you can rest well assured since he's above the title and you and the entire paying audience will be on line to get refunds if he ever calls out. That said, as others have mentioned, it's extremely rare to see an understudy go on in previews. They often haven't rehearsed and the actors use previews to tweak their performances as they see what works in front of an audience, which is why the director and creatives attend previews and furiously take notes. Obviously, it's live theatre and people get sick and crises arise, but there's no reason to expect you'll see an understudy.
Scratch and claw for every day you're worth!
Make them drag you screaming from life, keep dreaming
You'll live forever here on earth.
