A good overview of PHP5's new features can be found here:
http://www.devshed.com/c/a/PHP/Whats-New-in-PHP-5/
I particularly like SimpleXML, which lets me use XML data without sticking pins in a voodoo doll of Tim Bray, and the integrated support for Tidy, which lets me take user-generated HTML and clean it before it does anything ugly.
If there's a downside, it's the same downside that comes with any major rewrite: PHP5 is not as thoroughly tested as PHP4, which has gone through many cycles of bugfixing.
