Drupal has a substantial amount of good documentation; the documented code as seen on api.drupal.org, the developers handbook and even the installation and configuration guide has lots of good stuff in it. It also has older stuff that doesn't get updated in a timely manner and can be confusing. The last major handbook re-organization was Jan 2006 and that was a substantial improvement. Currently, just the three main books would be over 1,400 pages printed out.
One of Drupal's strengths also leads to it's frustrations. Drupal is flexible, it is content centric, so you can accomplish a given task in multiple ways. This is something that can be very hard to have people find the time to document. For those who haven't spent time writing HowTo documentation, it takes a lot longer than one would first think.
So, the problem, long term sustainable documentation with Drupal is difficult. Drupal core moves and changes with each version. For many versions, most of these changes resulted in only minor cosmetic differences in the user interface and menu placement.
With the release of Drupal 5, these user interface differences becomes rather substantial. JQuery's addition to Drupal in version 5 actually expands the UI possibilities in even more ways. With Drupal 6 we will see more interface and additions to core capabilities.
So what to do? At the Yahoo OSCMS I discussed this with a lot of people and came up with a new plan for the direction of some of the documentation and have been working on it since. For an identified base documentation we will use DocBook v5, commit it to CVS and version per Drupal release and generate PDF's in addition to having it live online in the handbook.