1) State of the Union addresses rarely have much effect on the president's approval rating:
By and large, Gallup has found that State of the Union addresses don't have much effect on a president's standing. The one exception? A 1998 speech by Bill Clinton. That year, Clinton's approval rating soared from 59 percent to 69 percent a week after his address. The speech itself touted the strong U.S. economy and came just days after news of the Monica Lewinsky scandal broke.
2) Presidents can get the public to care a bit more about the issues they mention in the State of the Union — but only a bit.
John Sides explains: "In an analysis of SOTU addresses from 1953-89, political scientist Jeffrey Cohen found that the more emphasis presidents gave to particular issues, the larger the fraction of Americans who considered those issues important problems. However, those effects were often short-lived, lasting at best a year in the case of economic issues."
3) At best, a president can increase public awareness about his positions — but the media needs to help out.
In a 2008 study (pdf), Florida State University's Jason Barabas found that voters were more likely to be knowledgeable about the president's positions on health care and Social Security if those issues were mentioned in the State of the Union address. But there's a catch: Voters only improved their understanding if the media actually discussed the policy details in the speech. And that doesn't always happen.
4) Fewer and fewer people are watching the State of the Union anyway.
In 2006, political scientists Matthew Baum and Samuel Kernell found (pdf) that fewer and fewer Americans were watching prime-time presidential addresses each year. The culprit? Cable television:
Even President Ford’s notably uncharismatic appearances did not prompt viewers to turn off their televisions (or tune over to public or local independent programming) offers compelling evidence that during the pre-cable era, watching the president imposed minimal opportunity costs ... Cable gives viewers choices, and for this reason, it makes watching the president costly.
That trend has continued with Obama: Last year, just 38 million people watched him deliver his State of the Union address. That was fewer than the 62 million who watched President Bush outline his justification for war with Iraq in 2003, and it was less than the 53 million who watched Bill Clinton deliver his 1998 speech.