I agree with the folks that have said it's up to the individual's taste's and needs. Here at my workplace, I've implemented not only Zend studio, but the new Zend Platform. There really is nothing that can compare when it comes to PHP server managment and debugging. The most useful features for us here, is the email notification when critical errors are triggered. We can log into the platform server, see the same screen/error the user saw, click on debug, the file opens in Zend Studio, and steps through the code. It's truely impressive. Zend's efforts to bring PHP to the enterprise are working.
We've been using the beta of Zend Studio 5, and it's support for SVN (subversion), code folding and intergrated PHPDocumentor are great new features. For us, it's been worth every penny, espcially since we're one of the only advertising agencies in the world using PHP as our application server (most are Windows/ASP.net or Java).
And finally, our developers use Windows, Mac OSX and Linux. Zend Studio works great on all platforms.