As someone who advises others on ways to increase their web traffic, including but not exclusively SEO, I use Alexa. Anyone interested in measuring web traffic should use it.
One thing that Alexa is great for is weeding out the hucksters. Most of us get email from so called SEO experts who promise to increase your SEO ranking. Take a quick look at their Alexa score and see if they can walk the walk.
Remember that the goal of SEO is to increase traffic NOT necessarily get ranked high on one or two keywords. For example getting ranked #1 for your company name on Google Search does not help your web traffic.
It is easy to get ranked high for one keyword but bit harder to increase relevant web traffic.
Alexa is an excellent tool for comparing website traffic in a general way in that it gives your website a ranking compared to all other websites. It is not great for measuring the website traffic for any one particular site.
For example I know what my site's Alexa score is and from Google Analytics and HubSpot I know what my site's traffic is.
Then, I can look at the Alexa rankings and tell if other sites are getting more or less traffic & if they are getting a lot more or a lot less.
You can register your site with Alexa for free and this will increase your chances of getting a more accurate ranking.
Paying for certification will give even more accurate results and give you tools to measure your site's activity.
For quickly measuring another website I use the SEO for Chrome extension for my browser and SEO Book has a great toolbar for Firefox: http://bit.ly/WffpYD The Alexa score is one of the basic measurements.
This is a great article about Alexa and everyone publishing on the web should know about their Alexa score, because everyone else knows your score.
Why don't you?
Download Franchise Info's Alexa toolbar which allow you to easily measure and compare every competitor's website to your traffic.
Start counting because your competitors are.
Great distinction between how to use Alexa and Google.
You need to know how to compare your website's traffic with competitors.
Measuring your own web traffic and competitors is important.
I use the Alexa as quick measure of service providers, competitors, blogger sites. It's a great tool.