I will post my recent experience on this matter:
I was rear ended at a slow speed while I was stopped at a traffic light which pushed me into the trailer hitch of the truck in front causing a small indent on front bumper and a 1/8" chip to the trim portion of headlight (replaceable triangle).
Called the other guys insurance and filed a claim, they had an adjuster come out and I pointed out any and all damage I could find, he provided an estimate on repair costs and provided a list of their approved shops.
They have their approved shops but you do not have to have work performed there but it does make any supplemental charge requests slower.
I took my car to the approved shop to have them take rear bumper and front bumper off to investigate if any further damage was done, which there was none expected or found.
But now for the good part ..........
The estimate was to replace bumper, the estimate was for a base bumper but I had the Aero Kit 1 bumper on so the approved body shop submitted a supplemental to insurance for added cost to replace which they approved.
For the Litronic headlight they also marked it as a replacement for entire headlight for $2600.00
I then told them I would just handle repair on my own with my body shop and for them to send me a check which they gladly did and also provided me with rental while car was in shop.
My costs to have indent fixed and entire front bumper resprayed (it needed it anyways) and to replace the headlight triangle: $923.00
The amount the insurance cut a check for was $5100.00 :dance:
As far as the "diminished value" if your car is over 5 years old and not a classic or high end car it does not fly here in Texas, California may be different.... but they may throw in some pocket change just to make you happy if you do complain.