It should be mentioned that, contrary to Ohloh's Analysis Summary, the development team of Drupal is very large. The stats are wrong, because many developers contribute to Drupal, but only a few developers have necessary rights to commit code into Drupal core repositories. This ensures a very high quality in Drupal's core code base.
That may sound odd, but it's actually the best structure I've ever experienced in a FOSS project. Because of that, it's pretty hard to find code in Drupal core that does not follow Drupal's coding standards.
For four years OpenConcept Consulting (http://www.openconcept.ca) developed its own CMS, but started making the switch to Drupal two years ago. We've been very happy with the switch and have been able to provide our clients with a much more reliable service.
Drupal has a great developer community, very flexible code base and a strong user base. The Drupal 5 has seen some huge improvements in usability (and 4.7 was pretty good). There is a lot of momentum behind this project and it is exciting to think where it will be two years from now.
Drupal is excellent open source cms, with lot of features "out of the box", very reliable, flexible and powerfull. Also has a lot of community contributed modules and themes.
So, more and more "big players" switching their web sites to drupal powered (www.mtv.co.uk, www.observer.com ...)
What can't you do with Drupal these days?
I can't give it 5 stars because it doesn't use an MVC model. Having html embedded and generated in the system files is poor implementation.
The admin interface css/html needs to be namespaced so that it doesn't override other styles. All css and html should exist only in the themes folder. Having to overwrite system css in the theme css file creates unnecessary redundancy.
Making these changes
Drupal had the right idea about content by using blocks, but I've never worked with such a disorganized mess of a system.
I hope they improve it, but once the framework is polluted its hard to clean up without doing a completely new version and rewrite of all of the core objects.
I've used Drupal off and on for a few years. They have a strong developer and user community and seem to focus on providing a simple, reasonably elegant CMS. It might not have all the bells and whistles possible, but it handles the basics very well and has a huge amount of contributed modules/add-ons.
Note that I wouldn't call it a WebCMS: on the one hand, it offers a lot more functionality than a CMS (calendar, poll, blog, ...), on the other hand it's no match for a real CMS like Alfresco...
But it certainly is very flexible with a lot out-of-the-box functionality (yet easy to set up and maintain), a driving community, lots of extra modules available for free, a nice API...
Current version (5.x) has some perfomance issues when combining some non-core modules and i18n / l10n, though 6.x should be a major improvement in this area.
If you're a developer then this CMS is for you. If you are not a developer then you can still use Drupal, but extending it beyond modules you download requires PHP knowledge.
I have been very happy with Drupal for over five years.
I can't imagine how to develop in a environment that haven't the backward compatibility like a priority with its users!!!
How to contribute with that speed and little stability?
I preffer Zikula CMS/WAF, the developers listen to the users! That's the order!