I had a recent discussion with my Allstate agent about insuring a '70 Cougar. She quoted me Haggerty agreed-value rate (alot!), and also the Allstate rate (for coverage similar to my daily driver), which was about half the Haggarty rate. I asked what the Allstate rate covered since there was a big price difference and was told "fair market value".
I think that is interesting, considering the wide variation of "value" in a '70 Cougar. A buddy of mine (with experience in this area) told me that they would basically low-ball the value of a "classic", and that you would have to prove your case if something truly horrible happened to your car. He did prove his case, but it took him several months before they cut him a check for only slightly under his "asking price".
When my '93 Cobra was stripped I pretty much had to do the same thing; prove the value of my car.
I think for "special" cars, this will always be the case.
Ok, I'm rambling, but just wanted to throw my tiny sliver of experience in there. BTW ~ I'm planning on going with Allstate regular old coverage, for now.