when using EV Search (or Archive Explorer) there does not seem to be an easy way to search for "Categories" that might be in use within Outlook.
as a workaround, it is possible to use the EVWebURL http://<EVwebURL>/search.asp?advanced and on "other attibute" use "keys" along with the value you want to search for. This does work as the Category field out of Outlook/Exchange already is being indexed.
This is not easy to use and train 5000+ users on this usage is almost impossible to accomplish in our case.
Therfore i wonder if those outlook categories could be added to the search website(s) (and probably to Archive Explorer as well) to give users an easy way to search for their categories.