Harlem is living out a new renaissance. After being the center of music in the 20´s it has been a forgotten area for almost one century. But one day, Bill Clinton bought one house here and things started to change. Today, the old mansions are being restored and the area is revitalizing again. There are cool stores, great restaurants, new hotels and interesting art spaces such as the Hispanic Society. Harlem is a beautiful part of the island.
You shouldn't miss a visit of a gospel church (try to worship, because you have the chance to see and to listen to a gospel choir). It's really amazing!
Correction: Bill Clinton's office has been in Harlem since 2006. He didn't buy a house there.
Though I'd agree that Harlem is currently undergoing a revitalization started in the 1990s, I wouldn't say that it has been a "forgotten area" since the 20s. It has always been an area rich in African American culture that suffered a period of decline, like the much of the city during a time, due to drugs and poverty. Luckily, Harlem is returning to its former glory. Stop by the Schomburg Center for a glimpse into Harlem's history: http://www.nypl.org/locations/schomburg