After an extremely long (IMO) release cycle the final version of PHP 5.2.0 was finally released yesterday morning. There are many new features, speed improvements and a fair number of security changes. You can read the
official release announcement for a quick summary of the major changes and the specifics can be found in a very long and somewhat boring to read
changelog
The bottom line is that all users of PHP 5.x should definitely upgrade and for 4.x users need to seriously start thinking about migrating as well, since we've finally got a 5 release that not only is feature complete but is also faster or at the minimum performs at the same speed as PHP 4.4.
Big thanks to all the contributors who made patches, reported bugs and ran tests to hopefully make a solid release.