That said, I've never had the problems that some people around here describe with upgrading. It's seriously just not that hard. Things just work, if you let them.
It really depends on the degree of customization and the plugins you are using. Not WordPress' responsibility, as many are quick to point out, but nonetheless this kind of extensibility is one of the major draws of WordPress. Various open source applications handle the issue of plugins differently, and with WordPress, it's a bit of a Wild West scenario...
(And PS, though my comment above was snarky, don't get me wrong -- I love WordPress, obviously. But the rapid UI changes and other version changes -- even when they are unqualified improvements to the product -- can be a source of frustration for a lot of people.)
I am not a scripter so it looks like I have destroyed my clients blog
I know a lot of people requested automated upgrades, but I really think it has the potential to be a dangerous feature.
Actually, I preferred how the older automatic-upgrade plugin handled it -- would turn off all plugins, backup the database and the source files, check for compatibility issues, and then do the upgrade.
Including an automatic backup of db and files beforehand is essential -- correct me if I'm wrong, but the built-in auto upgrade doesn't do this, does it? (Haven't used it yet.)
If it doesn't include auto-backups, there should at least be a big flashing warning -- don't upgrade until you have backed up your files and db.