The beach balls are fun if you throw them gently. But if someone throws one hard at your head from close range it definitely hurts. The park is clearly aware of the problem because one of the times the cast member gave the "how to fly your tire" speech he included a warning for people wearing glasses. Not that the warning does you any good because the beach balls come at you from all sides so there's no way to see all of the ones thrown at you.
By the way, I've been on Luigi's eight times now including three times after I got clocked by the beach ball. I really liked Luigi's the first time I went on it a month ago. It's only gotten more fun as I've gotten better at maneuvering. You can really get the tires moving if you play your cards right. And the better you are at maneuvering the more fun things you can get done in one ride cycle so it seems like the ride is longer. So other than the potential for minor contusions and damaged glasses I think Luigi's is a great ride. I plan on wearing old glasses while riding it in the future. And possibly bring a first aid kit.
I saw some chick out there wearing goggles. She may have the right idea.