Inspired by watching Amir Khella video "Keynote Prototyping Essentials" I played around with Keynote to see if it's really that simple. I have to say – it's awesome.
Reasons why Keynote rocks for prototyping webapps (don't know about other stuff but I guess it works nice as well):
But as in almost any tool there is a drawback (or lets call it a possible improvement).
So far I'm pretty happy with prototyping in Keynote and I'll definitely use it for some real client wok.