According to Google Analytics, this blog had 95 unique visitors yesterday, according to Extreme Tracker it was 149, and Site Meter tells me it was 129. Clearly measuring site traffic is an inexact science.
I agree with BL Ochman, measuring the time spent on a web page can be very misleading. The best system I have seen so far is Todd And’s idea of adding Google page rank, number of Bloglines subscriptions and Technorati links. The combination of those gives you some feel for a blog’s influence and authority.