I recently installed Drupal v6 on my computers at home. I actually did multiple installs as I wanted to see how the installation would react with prior content and starting from scratch. It didn't take me long to realize that I am probably going to have to implement some groovy database scripts to import only relevant content to the new site when it goes live.
A few years back I migrated 227net from Drupal 4.7ish to Drupal 5. This was a bit of a task and didn't really go as smoothly as was expected or described on the Drupal site. This was partly due to customizations I had made and partly because the whole upgrade process at the time was bit bit tenuous.
In 2008, Drupal released v6 of it's Content Management System. It is really about time that I upgraded but I am still holding back for a couple of modules to be ported to the new version. However, here are a few of my findings from the installations I have made on my computers so far. Bear in mind that I host on a custom WAMP installation on XP at home. My hosted sites are on Linux servers.
Clean Install
As detailed on Drupal v6 - First Thoughts a clean install was an absolute breeze. Graphically it is pleasing and it really doesn't take much to get a site running at all.
Upgrade
Following the instructions on Drupal, the upgrade procedure in essence should not be too hard. The main difficulties for me where:
Custom modules
PHP code
F**ked up categories
The first issue is not really an issue as such, however much relies on the second problem, PHP evaluation. Now there is, of course, the option to enable this but it appears that much like enabling blocks in different themes, you have to go and save the content again before the PHP evaluator actually kicks in.
This was not as much of an issue though as the categories - or taxonomy on my site. This has gone completely t*ts-up! I thought there was an issue presently as I tried to create URL aliases for each term - but they appear as nodes. This is not what was intended. Anyway, to cut a long story short there is a lot of unwanted categories.
I think that I will need to redo the taxonomy of the site - perhaps completely start again with the content. This may mean quite a few re-directs, but since my sites PR has dropped I don't think it will make that much difference at the moment. We will see how long this all takes to put into place. Hopefully the accessibility module can be upgraded and integrated at the same time.