$7,500
I haven't, but I helped one of my good friends do it when someone backed into his 1 year old Jeep Grand Cherokee a few years back. He was in a parking lot at a restaurant when a kid backed into him, hitting the right rear quarter of his Jeep. It was a low-speed collision.
After he had the car fixed, he took it to a body shop where they gave him a written report showing the repairs and how it affected the value. I forget the exact number but it showed a value impact of around $9,000 That report cost him $200-$300. He sent that to the kid's insurance company, and they told my friend they don't pay Diminished Value claims.
My friend then hired an attorney to send a letter to the Insurance Company. That started the negotiation process. He settled at $7,500 from the $9,000 estimate. I saw the check and got a steak dinner out of the deal.
The moral of the story is that (at least in Illinois) you can get paid for a Diminished Value claim but the Insurance companies will make you work for your money.