On NukeCops.com, the site of the Official PHP-Nuke development team, there's this post with a bit of their experience with the Zend Optimizer.

A friend of mine recently installed Zend Optimizer 2.1.0 as we have here at Nuke Cops. Both sites are high traffic (50,000+ page hits daily) and pound the web server. Zend Optimizer 2.1.0 increases page generation times at least 50%.

Prior to having ZO installed our PHP generation times were anywhere from 2 to 4 seconds consistent. Once ZO was installed and set to default configuration out of the box, generation times dropped down to roughly 1.1 seconds. Default config had an optimization configuration set to "15". Server load times actually decreased as well from a 1.x-2.x down to a 0.5-1.x.

They even offer a simple, little-known tip to help get your pages out even faster: in the php.ini file, change the "optimization passes" setting from 15 to 1023. This will enable passes 9 and 10 and squeeze in that little extra bit to get the pages out that much faster (unfortunately, this also comes at a price - higher server load).