I’ve tested the upgrade on one site so far, and it went very smoothly. On that site, I had customized my install.php file and during the upgrade via subversion, I was asked what I wanted to do with that customized file.
After the upgrade was installed, a database upgrade was required, but this was one click of a button.
Looking through the administrative interface for WordPress 2.8, the big things that jump out at me are the changes to displaying and handling themes, widgets, and plugins. I also notice the speed of the site – both the public pages and the administrative back end – is noticeably faster.
Also in the upper right-hand corner of the screen is a Screen Options pulldown menu, where you can change some of the way your post and page screens are displayed. If you change to one column, then change back to two columns, some items may not reappear in the right column. You can drag-and-drop those blocks from the bottom of the page to the right column.
Backing up your site before upgrading is always a good idea. Follow the issues on the WordPress.org forums as folks around the world upgrade to WordPress 2.8.
I’ll keep testing!