There's a clear favorite in this category. The world (at least the awards-giving-world) has fallen in love with the story of Rodriguez, a mexican-american singer-songwriter from the early '70s who made two albums that failed terribly in the whole world except for South Africa, where his music became the voice of a revolution. Searching for Sugar Man tells the story of the fans' search to know whatever happened to their mysterious musical idol. The movie is very well done and speaks very strongly to the idea of success, genius and recognition for one's talents. It's a wonderful story that has rightfully captivated most people who've watched the movie and has made Searching for Sugar Man the front-runner to win the Oscar.
The only thing standing between Sugar Man and the Oscar is the fact that there are other four nominees in the category whose subject matter may seem more "important" and "serious" than that of Rodriguez's story. I've seen two of these other nominees: The Invisible War, which is a harrowing look at the covering of rapes in the U.S. military and How to Survive a Plague, which serves as a document of the early days of HIV activism. The other two nominees; The Gatekeepers and 5 Broken Cameras are about the Israel-Palestinian relationship. As you can see, all very serious themes.
The bottom line is the following: the Academy may want to go with a very serious subject for this award (they tend to do so), and in that case, I guess the most probable winner would be The Invisible War. But whereas the winner of this category used to be voted by a panel in the past, the voting has been opened to all Academy members this year. And popularity is the strongest thing going in Sugar Man's favor, so expect it to win.
Will Win: Searching for Sugar Man
Should Win: Like I said, I've only seen three of the films. I liked all three a lot. If I had to pick one, I think I'd go with How to Survive a Plague.