Definitely depends on the team. It's can't be overstated for KC, Denver, Buffalo, St. Louis, etc. For others it's highly overrated: Indianapolis for instance, has basically the same record home and away since Manning has been the QB, but they're not perceived that way because of the NE playoff losses. Carolina is a team that I like better on the road if I'm betting against the spread.
Most people are really bad at distinguishing between how good a team is, and how big their homefield is. Seattle was a really good team last year, and they won every game at home, but I think it's absurd when people say it's the toughest place to play. If their homefield is so great, how did they lose to the Rams twice, the Cowboys, and get destroyed by the Bills when they were an average team in 2004?