What happens when you're working on websites that make use of PHP and are also very graphical?
In the past I tried using Zend studio, and when it came to building PHP classes and developing the bulk of a sites logic, it was excellent, however when it came to making changes to HTML/CSS I found it way more difficult to use that Dreamweaver (it didn't have a design view for one thing which helps). I found myself having to switch between both tools.
With Dreamweaver I find it's excellent for HTML/CSS, giving you at least a half decent impression of what the site will look like and whether or not your styles have been applied correctly, and it's also more than capable in terms of developing using PHP.
This may be my own impression, and maybe the wrong one, but I've found that things like Zend studio are only great if you're not really touching the HTML/CSS.
Everyone I know uses Dreamweaver, however speaking to another developer he was saying that everyone he knows uses Zend studio (or similar).