Used Aircraft for sale

How is the economy effecting used aircraft for sale? Is a Cessna 172 worth less than it was 5 years ago? Does the King Air still command the high dollar? Is the small business jet like the Cessna Citation or the Learjet still maintaining their value? Used aircraft for sale seems to be right with the economy. When the economy is good, used aircraft sales go up, but when it is down, used aircraft sales go down. So would buying a used aircraft in this economy at the low price, and holding on to it be a good investment idea?