A good friend of mine has been a lead designer at a number of big companies and he also uses Keynote to mockup UIs. For $20 it's not much of a risk to try it.

A good friend of mine has been a lead designer at a number of big companies and he also uses Keynote to mockup UIs. For $20 it's not much of a risk to try it. I like Fireworks much better than Photoshop for mocking up web or mobile UI too.