howdy cinq-
Not much to add, but I have friends who have used
ioncube and
sourceguardian which they liked much better than zend.
Zend "owns" PHP, so I imagine they know how to encode and optimize it well, but I think their main gripe against the Zend products was the price and dealing with the .zif files.
microcode is also a gnu php encoder that is gaining popularity that you may want to check out.