As always I have tried it out first, so here is my first attempt which was designed for a lecture in Technical Writing and communication.
The 'deal' is that you either let Prezi.com keep all your work as you work online for free, or you pay USD59 for the option of keeping your online work private, or you pay USD159 to download a desktop version of the programme. A final option - the one that I took - is to download the desktop version for a 30-day trial. This gives full functionality but leaves the Prezi.com logo on all your presentations.
Once again, though, it is probably only when you let your students loose on technology like this that you will learn what it is truly capable of.