I upgraded to WordPress 2.7 Beta 3 today. I used the “Upgrade Automatically” option from the Upgrade WordPress page. Very smooth process. All I needed to do was enter my FTP password and then wait for several minutes.
The first thing I notice is that the font for the navigation submenus has changed from serif to sans-serif. Beta 2 used serif for both the menu titles and submenus.
Other than that, nothing looks different; supposedly there are more than 160 changes in total. Find out more at the official WordPress blog.