I've was using Zend 2.5 since it came out and moved up to 2.6 once it was available. The new CVS integration was a huige boost for my ability to track my work. I love this IDE and wouldn't dream of using anything else.
It gives you the ability to use PHPDoc on the fly to documented your class & functions. It keeps track of of class, function, & variables scope really well. It does lose that ability when you are 2 include files deep in your code though. There are so many nice things that Zend does.
One little querk is that you can't delete files from the file manager. This is an extremely annoying issue, I wish they would have solved, but a small price to pay for all the other things you get.
I recommend Zend to anyone serious about their PHP.