Dmitry posted results of performance test comparing PHP 5.2 and 5.3 to internals which are impressive numbers:
Up to 30% performance win by simply updating PHP! Please help us with the quality assurance and test your applications using a 5.3 snapshot - we can't cover every usecase, you can!
And again as a sidenote: 5.2.6RC2 is out, please test that, too :-)