Now, I haven't quite made up my mind about Prezi. I've seen some really interesting presentations, but I've also seen a lot of seasickness-inducing efforts and since the latter outweighs the former, I've been struggling to get a sense of what I could do with it.
However... if you are interested in having more of a go with Prezi and you're a student or teacher... then have a look at the free Student / Teacher license upgrade option. All the stuff you can do with the free account, but an additional ability to make your Prezis private... remove the Prezi watermark... and get an additional 400MB of storage - all of which would normally cost you $59 / year. Gotta be worth an extra look, for that, I reckon!