Here's a look at how iAd Producer works:
iAd Producer's main screen shows the flow of your app. You can start from scratch but there are tons of "iAd Blueprints" templates that have prebuilt navigation and actions. There are two types of screens you can add: Pages and Shared Layers. Pages are, well...pages, and shared layers are reusable UI you can place across multiple screens (like a navigation menu).
iAd Producer is a drag-and-drop program so all your elements will come from the object library. The library provides default iOS UI but also comes with some fun stuff like 360 views, interactive drawing, maps, webGL, and more. Objects can be nested within each other (like the gallery I nested in a scrollview above) and are highly customizable. And if you still can't find what you're looking for, there's an HTML object that let's you write your own code.
iAd Producer gets animation timing *really* right. Recognize the UI? Keynote adopted it in its major update. If you open up the Action sidebar, you'll see all the various properties and Keynote-like animations you can apply onto object (they're all made from CSS-transitions). This includes keyframing your own animation. The timeline UI let's you to scrub through your animation or play back only certain parts of it, which is a real help for tweaking timing (and something woefully missing from Keynote).
In addition to animation, you can set up actions to trigger when an event happens on an object. In the example above, when I tap on this button it will change the color of my drawing brush to grey. Events can trigger actions like navigating to a different page, playing audio, or taking a picture. You can even combine a whole slew of actions/animations into an "Action List" and call that instead.
Who doesn't like CSS Filters? No one, that's who. Filters are available for all objects (this is how the DSCO filters were made) and they're animatable. 🎉Blur filters for everyone 🎉.
The most important part of a prototype is getting it on a real device so you can actually play with it. One of the best parts about iAd Producer is being able to download it to your phone using their iAd Tester app. You'll get some unneeded iAd UI with your prototype but it's definitely a step up from Keynote, where all you can do you is fake the interactions. It's also worth noting that you can preview your app in Safari and iOS simulator and utilize Safari's web inspector for debugging.
And there you have it! I think everyone should give this app a try. It's fantastically powerful and strikes one of the best balances between design and code I've seen in any app.