[amirmc]

Delightfully simple.

Additional benefit (not explicitly mentioned) is that you can test your UI/flows with potential users.

Imagine having a deck of cards with you in the pub. You could let someone navigate the app, card by card, and quickly identify the sticking points & user confusion. All without having to build anything.

[bane]

As an interesting alternative, I've gotten tons of mileage doing this in Powerpoint. You can even setup hyperlinks on the "buttons" to quickly move to different slides in the deck showing different interface states.