I used to use Pixntell for a simple, free option for narrated slideshows in the classroom, but now I'm sold on 30 Hands. I have used it with several students at the middle school level. It lacks a text tool (update: Text tool added as of 8/6/2013. Great!) and it won't let you pan and zoom, but for a quick narrated presentation or for digital storytelling it can't be beat.
(Another feature I haven't tested yet came with the last update. It allows you to duplicate a slide. The developer suggests using this for animations.)
Below is an example (created with the app itself) of the basics. I also included a second example where I talk about some games I've made.
The third video below shows how I transfer slides (usually created in PowerPoint) from my computer to my iPad using the Google Drive app. That process, in conjunction with 30 Hands, is a great way for students to finalize and publish their work.