Previously I used Drupal on this website, and it was always a pain to migrate from one version to the next – there were a number of hoops to hump through, things would often break, modules would need reinstalling and then after upgrading you’d find that some random bit of functionality no longer works (e.g. posting comments on a blog entry, or being able to see anything if you were an anonymous user).
So, when I saw WordPress 2.9 was out, I wasn’t overly quick to migrate from 2.8. Unfortunately I couldn’t tell if the 2.8 branch was still being maintained, and then when I noticed 2.9.1 was out, I thought I might as well make the leap (besides, it’s best to avoid .0 releases 
)
In my case, I run WordPress from SVN, as do many other similarly lazy people.
So, firstly to move to the 2.9 branch with Subversion :
svn switch http://svn.automattic.com/wordpress/branches/2.9 htdocs
svn update
Then, I visited my site – wow, it still worked. Logged in as the admin, and had a single button to click (‘Update database’). Milliseconds later, that was done, and it continues to work. That was easy. Where’s the broken stuff? Theme still seems to work, plugins are still working….
Worked pretty much the same for me too. Very slick. Now if only they didn’t insist on a MySQL backend…
The wordpress upgrading has been improving over the last few major releases. I’m totally sold on it. I tend to upgrade from the dashboard and haven’t had problems with themes/plugins for ages.
This may be a dumb question, but why do you run WordPress from SVN?
Dave – it makes upgrading very easy (‘svn update’) which I can script via cron on the server; so any vulnerability window should be quite small.
The alternative involves me actually visiting the site, or downloading a new release when it comes out – which is a manual process and time consuming.