Debates have decided the U.S. Presidency before. Kennedy vs Nixon, and Lincoln vs Douglas. When Reagan and Carter debated, Carter had a slight lead in the polls. With years of acting experience on his side, Reagan came across smoothly to the public and on Election Day he scored a landslide.
This proves that debates matter, and can certainly turn an election.
Obama and Romney: Romney has his work cut out for him in a debate with Obama. With his easy smile and intelligent answers, Obama can do debate. On the other hand, if Romney sticks to the facts and doesn't make claims the media can disprove five minutes later, he might have a chance. The temptation will be there for him to try and reach his base by exaggerating failures in the Administration and the economy. His better approach might be leaning more to the war angle, or any unfullfilled promises by Obama from the last election. Obama has more positive things lately to draw upon, with the economy finally beginning to improve.
It could go either way.
Obama will come with what he has, the way he usually does.
If Romney avoids hyperbole, he's in the running.
There is an early poll here. Feel free to state why you voted how you did.