At the level of the linked site, I'd have to say more "gimmick" than anything else. First of all, you still require a mouse to move around. At least, with conventional links and buttons, you can usually tab from element to element with the keyboard (no mouse needed). They're just doing Javascript, etc., rollovers on screen elements to pull down menus and pop up text. You have to be careful with something like this that you don't end up with "mystery meat" navigation, where you can't see at a glance where active screen elements are and what they're supposed to do. Also, how usable is a site like this for someone who is blind (using a screen reader) or handicapped (can't use a mouse, but could use a keyboard)?