I have a website that gets around 160,000 to 180,000 visitors a month and its ranked high around 190K.
Now I got another website that gets 1600 visitors a month and it is ranked in around 150K.
I am also noticing that tech sites or Internet Marketing sites do better then websites in other niches. I have websites that cannot crack a million, but do pretty good.
Now what I do find accurate on Alexa compared to 6 months or before is the clickstream (kind of scary) to learn your competitions top traffic sources.