Don't believe everything you hear about the housing market, unless it comes from someone who is qualified to make an assessment. If you have some need to know, like you plan to sell, interview a few agents.
either way the process is the same. Both the Realtor and the appraiser look at recent sales, the difference lies in maket value and "conservative probable" market value. Both are opinions and ususlly well founded. The only real way to know the market value is to get a ready and willing buyer to say they will pay X for the property.
As for the % of loss incurred year over year, that is a very generalized number and there is no set number that we can really go by. All I can say is that in my areas that I serve (generally Northeastern Baltimore County and all of Harford County), the market prices have definitely come down around the 10% range within the past year. However, they appear to be stabalizing recently and even starting to increase a bit (in certain price ranges). The price range that is getting the most activity in my areas right now is the $250,000 and under range. Generally speaking, in the Baltimore County & Harford County areas, this would basically buy you a middle of the road townhome, nice condo, or an older single family home that needs some updates.
You can certainly research your specific areas statistical data from http://www.mris.com and click on the market statistics page and then enter your zip code, county, etc..
Hope this helps a little.