Our experience in a grammar course here at North Seattle Community College
is that Reed-Kellog diagrams work quite well for about ninety percent of
the students.  THat is, the students are able to use them to analyze the
relations among the elements of a sentence.  For about ten percent, maybe
fewer, the diagramming can be a source of painful frustration.  Our course
also makes heavy use of sentence-combining (for putting sentences together
again, so to speak), and sometimes the non-diagrammers are able to shine
in the sentence-combining part and get a lot out of the class.
 
Where grammar instruction fails to carry over into better writing, I don't
think the diagramming, if it is used, can be blamed.  The main culprit, I
believe, is lack of time for practice in the application of grammar to
style. LIke so many maidens, the handmaiden of thought cannot be rushed.