Here's a tidied up excerpt of the current NEWS file with all relevant changes since PHP 5.0, which may help on the decision to upgrade (even from PHP 4).
Some of the things mentioned include:
- More than 350 fixes
- Some extensions moved out to PECL (11)
- PEAR upgrades
- Upgrade of the bundled Windows libraries
- Loads of Improvements and Addons
You can check out his post for the complete list...