I think that sometimes Alexa gets a bit of a bum rep. recently I have noticed that their data is becoming a lot more useful and accurate, read more on my blog: http://www.seoco.co.uk/blog/2007/07/08/alexa-data-coming-on-leaps-and-bounds/ and Digg it if you feel me.
I think Alexa is pretty bogus. But the fact is that they are about the only game in town for measuring traffic rankings for rank and file Web sites.
I don't think they're totally accurate, but they are useful as a guideline / vague idea of popularity - what more can you ask for really?