Fred Wilson has a fantastic post today discussing the critical subject of Website metrics, which remains one of the murkiest of online topics. He compares reports from several services and concludes that Google Trends may be a lot like Alexa, which means not very helpful except in the most general of senses.
He also notes that Comscore has big challenges if the traffic is below 500k (visits per month I think), which I think would exclude about 99% of all websites from getting accurately measured since that is a large traffic level.
His advice? Use lots of services and use caution when interpreting the numbers.