Its quick and it helps you to focus on the bigger picture and not the details see: http://gettingreal.37signals.com/ch09_Interface_First.php
especially this part: The Orange Pen That Started Blinksale
At work we had a dedicated UI designer, and he'd occasionally use Macromedia Fireworks for UI mockups. But that was primarily because our boss wanted professional-looking mockups to show to prospective customers. For communicating and iterating on ideas, I've yet to find anything that matches the productivity of paper. There were times where a week's worth of Fireworks mockups and meetings were resolved in about an hour when I brought a stack of paper into the designer's cube, took 5 minutes to sketch out a couple screens, and had him iterate from there.