“Many resources/libraries are available (often for free) to make visualization and mock ups even faster (e.g. Keynote KungFu, Keynotopia). Also when you combine this with tools like Glyphish you can have testable solutions quickly rather than getting bogged down in details.”
(Via Daring Fireball)
I don’t think Pages or Number are serious competition for Microsoft Office or Libre/Open-Office when it comes to complex documents or models, but Apple’s Keynote is a colossus of good presentation software and seriously kicks PowerPoint’s ass.
You should read the full article to understand how Keynote can be used for things you would never think using it, but see also the links on the segment above, there’s a lot of good resources for keynote there if you’re into designing.