Drupal 6 has been out for a while now, Drupal 7 is in the works, and yet most sites still seem to be using Drupal 5. Is it time to upgrade yet?
If it was just the core features the answer would be simple, Drupal 6 is better and is fully stable.
The problem is that many (maybe most) of the features of a Drupal website are powered by contributed modules, and modules written for Drupal 5 don’t work with Drupal 6.
This site uses 17 contributed modules; of these 9 have a clear upgrade path, 4 have new versions that are not yet stable, and 4 have no upgrade path.
I could write upgrades for the 4 problem modules (but I don’t really have time). I could decide I don’t need those features.
But Drupal 5 is still the only option if you want e-commerce, CiviCRM, and a whole host of other features.
I won’t be upgrading for now - for me Drupal 5 still provides the most flexibility.