Found it to be pretty good when looking to go a level up in fidelity from pencil and paper. The developers on tunedinapp.com seemed to like the results as it leaves a bit less to interpretation.

The built-in components are great if you're using mainly the standard iPhone look. My only drawback would be the lack of a 'master' slide with the iPhone in the background, but it's easy enough to put together.

Certainly worth a few bucks to see if you and your team like it.