Part of what I do for a living is teach internet programming with tools made by Allaire. Unfortunately, like many Windows GUI's, the development tool sports dockable toolbars. These provide no end of frustration during classes: 1. if you move your toolbars, they don't match the instructor's machine and make my directions difficult. 2. Undocking a toolbar makes it float. Since this action is rarely intended, the user attempts to go on about his/her business and clicks back in the main interface and effectively loses that piece of the interface. Joel, with years of experience at M$oft, explains the source and pain of this bad interface idea.
Some HCI specialists think that interface customization is just a bad idea. A slightly different camp argues non user directed, or adaptive interfaces are bad. I'm more inclined to the latter, and actually like, most of the time, the adaptive start menu in W2K (I don't use Word enough to know).