I use Drupal as my primary development tool for websites.
After many years of creating customized content management and ecommerce solutions, I started searching for a better way to create sites. I tried Mambo, Joomla, Expression Engine, OS Commerce, Zen Cart, and dozens of other systems. None seemed to have all the features I was looking for. I wanted an Open Source product that was powerful and flexible enough for any kind of project I worked on. More importantly, it had to be stable, secure, well-maintained, and built by professional developers who were aware of the problems that a less than perfect system could suffer from, including peer-reviewed code.
Unlike many of the tools I found, not only is Drupal feature rich, it is also built with an extremely high level of professionalism, more so than many commercial products I have used. Anyone can contribute to the project, but to be taken seriously requires a considerable investment in experience and helping improve the project. There is a great deal of code review and regression testing involved before any new features are accepted. Individual modules and themes are constantly being improved, not just to add new functionality, but to also ensure high performance, flexibility, and (perhaps most importantly) a high level of security.
The beauty of Drupal is its flexibilty; one can create a site right out of the box, leverage the strength of thousands of contributed modules (which add functionality) and themes (for visual presentation). And if you need to add more functions or customize the look of the site, the possibilities are endless, with its rich and well-documented API.
Drupal scales well, from the smallest brochure and information sites to huge sites with great amounts of data. Drupal is also becoming popular for government sites.
And some very big players are using it (Tesla, GE, Whole Foods, McDonalds, etc.).
And if that's not enough, Drupal is free. Like any Open Source project, developers share what they have learned with others and contribute back to the project as they expand its power.
One of the important parts of learning and using Drupal is actively participating in the community. I have been a member of drupal.org since 2007 and have contributed some module; see my profile at the project site. I am also a member of the Sacramento Drupal User's Group.