That is the one downside of copying files, it can take a while to search and copy. Since most people put documents either in the "My Documents" folder or on their desktop that is where I would have it start for backing up my files :-), otherwise you will probably get a lot of crap, like product documentation etc. What would be really slick is to check if either Google or Yahoo desktop search is on the system and query the API, that would probably make it go a lot faster.
I use a similar script to back up contents of some folders I keep code, although a bit different in that I compare the contents and have it update any files that are new and archive/rename the older versions.
I put up a description here on the "slackers" guide to podslurping, with a quick downloadable package.
http://www.usbhacks.com/2006/10/29/how-to-...sb-flash-drive/
Not sure if this is the kind of thing that would go on the Wiki here, it is pretty simple really, folks here could probably do something a lot better, especially if it is written in a lower level language instead of relying on a batch script.