From the 2bits.com site today the results of a recent test have been posted - comparing a Drupal installation's performance on a normal Apache/PHP fcgid install to a new install using the Zend Server Community Edition.
Zend have had their own proprietary PHP server product for quite some time now. This week, they announced their Zend Server Community Edition, with the intention to disrupt Java, and grow PHP specially in the current economic conditions. [...] So, we decided to benchmark Zend Server Community edition with Drupal 5.x against Apache PHP fcgid with APC, which we have benchmarked recently.
They used Siege for their benchmarks, testing the response times on both installs with one coming out a good bit ahead of the other (I won't spoil it, you'll have to see for yourself). Future testing is mentioned for other comparisons like Zend mod_php vs Ubuntu's Apache mod_php and others.