It seems prudent to get this out of the way up front: no, the film's trailers and title are not misleading. The entire 90 minutes of Rodrigo Cortes's "Buried" does indeed take place in a coffin containing Ryan Reynolds, who plays an American contractor in Iraq. He has been kidnapped and buried alive for ransom.
Whew! That feels better.
"Buried's" very essence is a gimmick, and there really isn't any reason to beat around the bush -- if the gimmick works for you, the film is going to be a win. If it doesn't, you're not likely to be won over.