Although its not truely accurate, its really not bad for gauging traffic. I use it for my sites to see how they fair against competitors and such. Ive found that when my traffic spikes (low or high) it is reflected on Alexa. So I wouldnt complete discount the site, it is still a valuable tool.
However there is a service out there that is more accurate. I believe its a paysite and right now the name eludes me. But I had this conversation with a friend over traffic ranking and he made me aware of it. Keep digging, someones gotta know what service it is.
Justin