Product review (3 stars): Fix for breaking knobs.
Like many, I have purchased these multiple times because the design is poor.  The sleeve that attaches to the burner control stem breaks.  Many have suggested that Jenn Air should make these in metal, rather than plastic, but that is apparently prohibitively expensive.  I've looked into having a shop make some out of aluminum, and also into having some 3D printed, but both of those approaches are also expensive.  I have worked out an inexpensive addition to this part that I believe will add enough support to the sleeve to prevent breakage.  In short, add a supporting band to the plastic part.  The outer diameter of the plastic sleeve is approximately 7/16ths.  There are many ways to add a supporting band, but a readily available part is a delrin compression sleeve made for plumbing.  Use a 3/8" OD delrin compression sleeve.  Fit the compression sleeve over the plastic part and slide it down the shaft.  Add another compression sleeve.  (Use a glove, as the fit is tight and getting the compression sleeve onto the plastic part requires some force.)  Once the compression sleeve is onto the plastic shaft, press knob down onto a flat surface to bring the compression sleeve flush with the end of the plastic shaft.  A 3/8" 12 point box end wrench may be used to press the compression band further down the plastic shaft.  Add a second compression sleeve.  The two compression sleeves should provide enough support to prevent the failure mode I most often see, the plastic sleeve splitting and breaking.  This fix will not prevent someone from forcing the knob to rotate (without pressing the stem spring down first) and stripping out the plastic.  But, breaking the knob should be much more difficult after this addition.  Something to consider...the softening temperature of delrin is approximatley 150 degrees Celsius or 300 degrees Fahrenheit.  The area near the control knobs on my stove never gets anywhere near that hot.

For all those who are tired of buying these expensive plastic knobs over and over again...you're welcome.  :-)