I've been using Prezi for a while now, using it for classroom presentations and presentations at conferences.
I found Prezi very easy to use and fun. You can easily create a presentation from scratch, adding text, images, video, links and more. It is very easy to modify them all also with just a click. You can also import slides from a PowerPoint file. The presentations are very dynamic and you can really make them more visually engaging and interesting than PowerPoint or Keynote. It also moves people away from those dreaded bullet points.
The site also has great help resources, including videos, a help site, and more. There is a community of users on Twitter, Facebook, and the Prezi forum also.
You can get free accounts for educators or the public. There are limits, but they haven't affected my use at all.
You can create your Prezi's and present them online, or download them for offline use. You can also use Prezi desktop software (with a paid account) to create Prezi's offline.
There is even Prezi Meeting, where you can edit and show Prezi's with up to 9 other users. Work collaboratively on a Prezi or use it as an online meeting platform.
It's easy to get started and there are a ton of Prezi's that you can look at as inspiration.
You can make your Prezi's private, public (viewable by everyone) and public with the ability to copy.
Prezi is fun, easy to use, and create's some great presentations. Try it out.
Example of one of my early Prezi's (overview shot): (done with minimal experience with Prezi)
Here's the link to the actual Prezi: Google for Educators