Here's a few quotes from others describing the experience:

Where all your dreams are realized (as long as you dream of rolling down a hill inside a giant plastic ball). Where the chance to mutilate yourself and die a terrible death while screaming your lungs out is replaced by the chance to partake in a totally bizarre and fun experience protected by a MASSIVE cushion of air with no chance of mutilation or death while screaming your lungs out. (http://www.zorb.com/main.html)

Zorbing is slightly less intense but sillier: A person is harnessed inside a nine-foot-wide inflated plastic sphere and sent rolling down a hill. If you've ever wondered what it would feel like to get stuck inside a washing machine, try zorbing with friends inside a harness-free ball that's filled with enough water to make everyone slide around (and become better acquainted in the process).

'Who in their right mind would pay to roll down a hill inside an inflatable white plastic ball doing something dubbed 'zorbing'?" Luba says. "It's a mystery, until you see the giddy, grinning folks emerging from these contraptions. Then pretty soon you find yourself standing in the ticket line. The pointless goofiness of these activities appeals to the child inside us all." (http://www.rudymaxa.com/article.php?ArticleID=117)