What are some alternatives?
Marvel - Turn sketches, mockups and designs into web, iPhone, iOS, Android and Apple Watch app prototypes.
Form - Instant Native Prototypes. Form lets you build custom native prototypes directly on the device.
AppCooker - Design, mockup and prototype mobile applications with AppCooker. Discover the most powerful mockup engine ever created on the iPad. AppCooker and AppTaster deliver the full scope of the prototyping tools.
Balsamiq Mockups - Using Balsamiq Mockups feels like you are drawing, but its digital, so you can tweak and rearrange...
POP (Prototyping on Paper) - POP helps you make interactive prototype with ease. If you can draw, you can design apps.
MockFlow WireframePro - Super-easy wireframing