i went to DW for the first time in 2000 and we routinely did walk=ups for meals. Over the years, however, it's become very difficult to do a walk-up and, as was mentioned above, the very popular places that you might really want, are nearly impossible to walk up.
I would love to be able to do it on the fly but if you want something more than a hot dog these days, ADRs are a must.