Alexa can be a good or bad tool to measure traffic. If not bamboozled with it can be good. But many webmasters use software like Alexa Booster and it will give false results.
Even Google PR im seeing has it flaws. Google seems to put alot of emphasis on Backlinks, more then genuine traffic. You can have sites with little to no traffic with tons on Back’s have a PR4-5. Sites with good traffic and little links are still at 1-2