okay I think I got it.
the built in filter function doesnt work for this
or it is not clearly explained in the documentation on how to set it up for IS NOT NULL
If you want to filter a list and ONLY show users who HAVE filled out a particular field.
Then in the filter section of the list edit page
select the advanced radio button.
then add:
`cb_desiredfieldname` IS NOT NULL AND `cb_desired fieldname` != ""
This will only show the users who HAVE filled out that field.
usage example:
If you are using CB subs, and a each sub level only has access to certain fields then this is an easy way to grab those users and display them in a list.
I wish the documentation manual was more detailed about the IS NOT NULL subject.
I found the answer fully posted here
www.joomlapolis.com/component/option,com_joomlaboard/Itemid,38/func,view/id,108501/catid,77/limit,6/limitstart,0/