My new favorite billboard on my gruellingly long commute is one for Realtors. Its says, "Is now a good time to buy a home? Ask a Realtor." And of course, this billboard was put up by some sort of association of Realtors.
It's unfortunate, because they probably are some of the most knowledgeable people about that question. However, I don't care who you are, if you have a vested interest in the answer to a question, I'm going to be have a hard time taking your advice. Realtors only make money when they sell houses. No houses selling, no money. Who in their right mind is going to tell you not to buy a home if their entire livelihood (today a very battered livelihood) is 100% dependent on you doing exactly that? If I sell apples and someone says, "Is now the time to buy apples?" It would be stupifying for me to say, "No, now is not a good time to buy apples." Of course I'm going to tell you to buy what I'm selling, just like the Realtors are. And it's unfortunate, because there's a good chance their advice is good, despite their built-in bias.
I think everyone needs to take a course in third-party credibility.