Do you have to be using one of Google’s services such logged into google (gmail etc) or using their toolbar or a site owner with google site-maps or using analytics for them to monitor/register visit numbers to a particular site (like Alexa)? Can’t see how they’d know otherwise? Though that all does cover a lot of people
Sorry I meant to say if you want to see daily estimates of visitor numbers, the really useful bit, you will need a gmail or other google account and to be signed in.
On the point of how accurate their figures are, well thats open to interpretation. They use a mix of Analytics and other data (toolbar, clicks tracked in search results etc). They don’t promise its foolproof, but its the best single aggregation of global web data available. Prior to this the best provider of this kind of data was usage data from ISPs which was notoriously parochial and skewed. And expensive.
Short answer: I think its pretty damn accurate.
Without hijacking this post anymore, and for the terminally geeky read my blog post about the subject: http://millsbomb.blogspot.com/2008/06/competitive-intelligence-google-gives.html