Alexa ratings are generated only bu people who have this toolbar installed. That tends (not definitive, but tends) to be power users, not the casual or less intense user. Thus, ratings are skewed and useless.

This is SPY ware

Listed as Spyware by Spybot - Search and Destroy 1.3!!! Mixita - Anti-Spyware Team

I've met a lot of people who use Alexa data (e.g. Web site rankings, reach, etc.), but I've never actually met anyone that used the toolbar. It makes me a little suspicious of Alexa's claim to have 10M installed, which makes me doubt how accurate their rankings are. It's kind of a shame that more people don't use the Alexa toolbar, because the Wayback Machine is kind of cool - it lets you see screenshots of what your favorite Web sites looked like in past years (rateitall looked pretty funny - anybody remember 'one world, many minds?'), and the related links tool is also pretty helpful for finding similar sites. Unfortunately for Alexa, they just don't have that killer app like a Google search, or Yahoo messenger, that will cause people to think that they need it. Neat technology, less than compelling application.