Well, I (finally) decided to see what the Hill's site had to say
about the c/d (and s/d, if I remember that aspect).  That because
I was looking at Wysong's Uretic formula, and that indicates it
helps prevent problems and says nothing about dissolving
crystals.  Since some folks have said that the c/d dissolves
crystals, I thought that might have been an important
distinction.  Well, here is what the site says about c/d:

Dry:  "For the Nutritional Management of Cats with Struvite
Related Feline Lower Urinary Tract Disease (FLUTD) .

"FLUTD is a condition often associated with the formation of
crystals in the urinary tract that can cause numerous painful
symptoms for your cat. The two most common types of crystals that
form in the cat's urinary tract: struvite and oxalate.
Prescription Diet® Feline c/d is formulated to provide nutritional
management of cats with struvite-related FLUTD. Prescription Diet
c/d contains reduced building blocks of struvite and produces an
acid urine pH, lowering the chance that struvite crystals will
form. The nutritional formulation of Prescription Diet c/d® may
also be useful for pets with a variety of conditions."

So, this sounds like it is for maintenance and not for dissolving
the crystals.

BTW,  the canned c/d with chicken has the following ingredients:
"Pork by-products, water, pork liver, chicken, chicken fat
(preserved with mixed tocopherols and citric acid), rice, oat
fiber, corn meal, starch, corn gluten meal, guar gum,
DL-methionine, taurine, brewers dried yeast, minerals (calcium
sulfate, dicalcium phosphate, potassium chloride, calcium
carbonate, salt, zinc oxide, ferrous sulfate, copper sulfate,
manganous oxide, calcium iodate, sodium selenite), beta-carotene,
vitamins (choline chloride, vitamin D3 supplement, vitamin E
supplement, thiamine mononitrate, ascorbic acid(a source of
vitamin C), niacin, pyridoxine hydrochloride, calcium
pantothenate, riboflavin, folic acid, biotin, vitamin B12
supplement)."

c/d with seafood:
"Pork by-products, water, pork liver, fish, oat fiber, chicken fat
(preserved with mixed tocopherols and citric acid), rice, starch,
corn gluten meal, guar gum, DL-methionine, taurine, brewers dried
yeast, minerals (calcium sulfate, potassium chloride, calcium
carbonate, salt, zinc oxide, ferrous sulfate, copper sulfate,
manganous oxide, calcium iodate, sodium selenite) beta-carotene,
vitamins (choline chloride, vitamin D3 supplement, vitamin E
supplement, thiamine mononitrate, ascorbic acid(a source of
vitamin C), niacin, pyridoxine hydrochloride, calcium
pantothenate, riboflavin, folic acid, biotin, vitamin B12
supplement)."

c/d, unspecified flavor:
"Beef by-products, water, pork liver, corn meal, pork by-products,
corn gluten meal, powdered cellulose, xanthan gum, locust bean
gum, taurine, brewers dried yeast, iron oxide, minerals (calcium
sulfate, salt, potassium chloride, dicalcium phosphate, zinc
oxide, ferrous sulfate, copper sulfate, manganous oxide, calcium
iodate, sodium selenite),beta-carotene, vitamins (choline
chloride, vitamin D3 supplement, vitamin E supplement, thiamine
mononitrate, ascorbic acid(a source of vitamin C), niacin,
pyridoxine hydrochloride, calcium pantothenate, riboflavin, folic
acid, biotin, vitamin B12 supplement)."

Now, compare this description of s/d:  
"Feline s/d®
"For the Nutritional Management of Cats with Struvite Related
Feline Lower Urinary Tract Disease (FLUTD).

"FLUTD is a problem often involving the formation of mineral-based
crystals and stones in your cat’s urinary tract that can cause a
variety of painful symptoms, including obstruction. Struvite and
oxalate are the two most common crystals that form in a cat’s
urinary tract. Prescription Diet® s/d® is formulated by
veterinarians to help dissolve struvite crystals and stones in
your cat. Prescripton Diet s/d has low levels of magnesium and
produces an acid urine pH to aid in the management of struvite
crystals and stones. The nutritional formulation of Prescription
Diet® s/d® may also be useful for pets with a variety of
conditions."

I take these descriptions as being "from the horse's mouth".  So
c/d is for maintenance, while s/d is for dissolving CRYSTALS and
stones.  NONE of the vets I consulted with had this right.  And
the s/d was only given to us as something to try to see if Mingy
would deign to eat it.

I think I'm going to be as disillusioned with vets as I have
become with doctors...........................