Some organizations have experienced, unbiased research professionals on staff, and do reliable work in-house. Whenever possible, try to access the original study from the research firm, rather than relying on quotes from the sponsoring organization or from an article. That way you know you're getting the whole picture. Without this caveat, Barna's figures could easily be misused in an article or a press release. With the original study in hand, you have the full story.
Some national attitudes change very slowly, while others can change almost overnight.
Because a study is three years old doesn't necessarily mean it's inaccurate. For example, the proportion of Americans who own a VCR won't change much from last year to this, because it's a mature technology. But the proportion of people who access the Internet will change drastically in a year's time, because the technology is new. But political attitudes can change month to month. Look closely at when the study was conducted, and what might have changed in the interim.