Use of GluUnproject

Has anyone have any experience of using the above routine. It is supposed to map any windows screen coordinate to real world coordinate.

GluUnproject winx,winy,winz, modelviewmatrix, projectionmatrix, viewportmatrix, objx,objy,objz

Regardless of what values of what I use for winx, winy, winz, I get the same values back for objx, objy and objz.

Where am I going wrong??

Many thanks