Useful for what, worthless for what? is the better question. While it is an inaccurate gauge of traffic for small sites (as those stats can be easily manipulated), it becomes more reliable for quickly identifying the top 50,000-100,000 sites (which are not easy to manipulate). Alexa is not useful for SERP or SEO purposes---meaning it IS useful if you're not pursuing SERP, or ranking in the search engines for traffic.
As a famous thread by Ferma here showed, it is useful approximate tool for identifying the top sites traffic-wise for use in setting up link-thru traffic. There is another game in town for getting traffic than Google, and a tool doesn't have to be precise to be valuable. The "it's worthless" chant on this thread represents a SEO-based marketing bias.