The students I teach at Solent University seem to believe that Alexa is a reliable and trustworthy source of traffic figures for sites on the Internet. In reality, the data it provides is reasonably accurate for highly-trafficked consumer websites. But anyone who has tried to use Alexa to estimate traffic levels for B2B technology sites will have realised the limitations of this tool.
The problem with Alexa is the limited sample size (users of the Alexa toolbar). If you’re not familiar with Alexa, or if you think it’s a reliable tool to estimate website traffic in our industry, I’d strongly recommend that you read “Debunking the Myth of Alexa Rankings”, a great article in EE Journal.