well as they only access people who have the alexa toolbar installed.They have very little relevance to true traffic values.
Especially as there toolbars will attract certain types of people and certain areas of products.
Alvin
That said, I still think it's useful for comparing yourself to your competitors, as you will likely have similar customer profiles and demographics - and therefore a similar percentage of visitors who have the toolbar installed.
I certainly use the toolbar in this way, but would take no notice of stat comparisons between different types of sites/sectors.
At the end of the day it's the only tool of it's type, however flawed