Having started in IT in 1986, I've seen the entire gamut in terms of requirements gathering - from 5" thick requirements documents to high fidelity simulations with iRise to now... low fidelity simulations with Balsamiq. You might wonder: why use a tool that generates simulations that look like they were scrawled on a napkin?!
Based on previous experience, when you have high fidelity simulations, it's easy to get caught up in focusing on the wrong details: wrong color, icon is 4 pixels too far to the left, etc... before anyone has even validated whether or not the application is functional, understandable, intuitive, and easy to navigate.
Balsamiq is easy to use, and abstracts away these potential distractions. I've used it to build the simulations for my first iPhone app and found it effective so far.