You don't need CB Query Field for that. Add [user_id] substitution as the description to a Custom HTML field (Delimiter field if on CB 1.x) and it'll display the users id. Please see the below tutorial for further substitution usage information.
www.joomlapolis.com/support/tutorials/107-use-cases/18353-using-substitutions-throughout-cb
BTW, what is the difference between the fields 'id' and 'user_id'?
None, but they should always be the same. All queries, etc.. should use the id column though as eventually user_id will likely be deprecated.