George is right on for application to appraisals. No index was invented to be applied for an individual appraisal assignment.
Even in the San Diego MSA, we have some (damn few) "neighborhoods" that are relatively flat or stable market values. The absolute majority of neighborhoods are declining market values.
However, the CS index for San Diego is a valid measure as to what is happening here on a wide scale. If you are a bank and made loans in 2006 all over the San Diego MSA, you can guess that your portfolio has major losses. If you are a home builder in Carlsbad with unsold inventory today, chances are you will have to seriously drop the prices to compete with the oversupply of housing on the market.
The value of the CS indexes is that you can buy or sell future contracts on specific MSA cities. If you are a builder, you can hedge your exposure by selling the appropriate contract. If values go down, you win, you can offset some of your real losses on homes that you have. If you are a buyer of mortgages, you can hedge the losses you are sustaining from foreclosures.
The value of the CS index is that you don't have to be "right" about a specific house value or specific neighborhood value in the MSA. Another value of the index is that you can track the futures contracts to see where values may be 6 months from now (Wow! a forward looking price indicator).
See the attached graphic of the CS index for San Diego with future contract data. People with real money are betting for more declining values to come.