I've been running a small test between three menu systems over the past couple of months.
Popout and Dropdown Style Menu via CSS
Popout and Dropdown Style Menu via Ajax
Here is what I am running into. While the CSS Popout and Dropdown menus are cool and search engine friendly, they may not be all that user friendly.
Of my test group, 90% preferred the collapse and expand style over the other two methods. The biggest complaint with the popout and dropdown style menus was "targeting" and "remembering" where you are.
While the popout and dropdown style menus are compact, I believe they do present an issue for many who just can't figure the damn things out.
What's your take on the above menu styles? What method do you use when you have upwards of 500 categories with thousands of products?
Note: We're using a combination of the expand and collapse menu along with a linked breadcrumb that follows taxonomy. This provides the users with two access points. Also, my test group are site administrators and not regular site visitors. Our site visitors get a dMoz style category structure. We're debating whether or not to unleash the Collapse/Expand system to the public.