tedster - 6:20 pm on Jul 2, 2010 (gmt 0)
Alexa gives a very general and fuzzy picture - so do the other "outside looks" that are available around the web. I use them as just one of many looks at the competition on a SERP. It would be a lot better if they would just leave their logs open ;)
In the cases I mentioned, the traffic differences are about three levels of magnitude - over 1,000 times smaller for the negative site. I think Alexa and the others are more accurate than that, so I do take it as having some value.