I discovered Prezi last summer though a blog, iirc. It was right when I was preparing presentations for job interviews, so I thought I'd try using Prezi instead of boring old PowerPoint. And I found that Prezi was actually really easy to learn (their interface is quite intuitive), and it impresses people who haven't seen it before. Because it looks all cool and animated.
In my opinion, the worst Prezis are those that just take a Powerpoint presentation and paste the content into Prezi. Like this Prezi about the history of the Connecticut State Library. Everything is the same size and same orientation. You just go from slide to slide, and it looks little different from a normal PowerPoint presentation.
The good ones make use of Prezi's zooming capabilities to make the presentation more dynamic. You don't want to make your viewers seasick, but you want to let them swoop through your info a little. Just for fun. I like this Prezi, which contains info about a Summer Reading Program at Lynnwood Library.