I almost never leave a review but I think that there are better or cheaper apps for prototyping... I logged in and am leaving a review because I think everyone should be aware of two great alternatives.
For advanced devs {{and $10 bucks cheaper and just more accepted by the dev community}} there's BALSAMIQ MOCKUPS ...and they've been around a long time and NOT going out of business like say Compòse... which was just too expensive (especially with the extensions).
For beginners - and my personal favorite - there's MOCKS by the same developer of the famed Fluid browser, Celestial Teapot... MOCKS is a great prototyping app, super easy to use and I just really love this app... especially at only $10. dollars! Great Fun, Cool & Cheap.
Flinto is just the newest thing, not available for older OS's and too pricey in comparison to the alternatives.