I'm a big fan of Prezi, and think that impress.js is pretty sweet, but I have to admit, every time I try to dive in and use either tool to build a presentation, I realize how much planning is required to execute one effectively.
I'm still a huge fan of reveal.js[1] and love the fact that I can get a rich presentation, with all the functionality I need in an easy-to-use, extensible framework. Ever since discovering it, I haven't been able to present well with anything else...