1) These are still just estimates. Without someone totally sharing their traffic data with you, you can't ever get a precise read on someone's traffic. That being said, any inaccuracies should be comparable across most sites, so comparisons of two sites should still be pretty spot on. 2) Unlike the aforementioned traffic rating sites, Google has one big advantage; they're Google. And being Google, they have access to all the data they choose to share with themselves. According to Google, Google Trends for Websites "combines information from a variety of sources, such as aggregated Google search data, aggregated opt-in anonymous Google Analytics data, opt-in consumer panel data, and other third-party market research..."

This is an example comparing FindLaw.com to Lawyers.com. FYI, the graph is showing DAILY UNIQUE VISITORS for each site. Click the picture if you want to see this analysis "live" on Google. I posted just the past 12 months, but you can pick from a variety of time-frames.

Tucker Carney