A small study of traffic rating sites by Seomoz.org concludes:
Based on the evidence we've gathered here, it's safe to say that no external metric, traffic prediction service or ranking system available on the web today provides any accuracy when compared with real numbers. The sad conclusion is that right now, no publicly available competitive analysis tool we're aware of provides solid value.
The survey evaluated 25 SEO blogging sites, and might not reflect the overall situation for conventional retail sites. Nonetheless, these data do give one pause regarding the quality of traffic numbers from the ranking services.