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 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.
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!