In checking James Buchanan's page on wikipedia, I came across this information about him after his term ended.
In 1866 Buchanan published Mr Buchanan's Administration on the Eve of the Rebellion, the first published presidential memoir, in which he defended his actions; the day before his death he predicted that "history will vindicate my memory."
Still waiting for that vindication. Whether you're considered a "good" or "mediocre" president isn't up to the president. It's up to historians, looking at what happened during and after an administration. Truman's standing has risen since he left office in 1953. Where Obama will fall depends on his actions, not his words.