I use Google Insights quite often. One of my business models is setting up lead gen sites and selling the leads for a flat rate, commission of job, or lead packages. When I find a market I want to target, I use Google Insights to drill down to the most popular location, then use that data to put together a lead gen site for the location that Insights told me the market is hot and ripe for the taking. If you don't work as a Marketing Consultant or Lead Gen wiz, Insights can still be useful. At times, Insights will give you search data that standard keyword tools don't give you - like which search terms are "breakout" terms, meaning they have become popular search queries in the past few weeks/months.
I've heard that it's a great tool, I've never used it personally though. If you have the time I would suggest tryin it out!
I´m using Google INsight whenever I research new niches or micro niches, to determine the searhc volume trend and avoid putting work into "dead" niches! It´s also a great tool to detect low competiton keywords not showing up in the Google Keyword Tool!