Upgrading from a previous incarnation of version 1.5 is simply a case of backing up the MySQL database, backing up the current WordPress installation, deleting everything apart from \wp-content and wp-config.php, and then uploading the new files. That’s it.
WordPress 1.5.2 contains a number of security fixes, bug fixes, and a new “Save and Continue Editing” button for Pages (although I’ve just noticed that this is only available once you have actually created a new page, which I guess makes sense).
I love WordPress. It’s simple to install, use, develop, and upgrade.