Legal Theory Blog has this post evaluating the arguments for against judges with "mediocre" intellects.
The discussion reminds me of a story I once heard told from the pulpit at Holy Trinity Church, in Georgetown (the church where I was married). The homilist was some fellow from the Church of England, with a syrupy British accent and bone-dry sense of humor. (They had many interesting guest speakers.) He related the story of a young priest assigned to a class of teenagers, who made the mistake one day of opening the floor for questions, and the first question was this: "why are the clergy so mediocre?" He thought a moment, and answered, "because we have only the laity to pick from."
It seems almost miraculous to me that judges are not more mediocre, given the role of politics, the financial sacrifices, the soul-baring required for confirmation, and yet, as Judge Posner wrote in the second edition of this book, "I may be living in the golden age of the federal appellate judiciary. There may never have been a time when so large a fraction of federal judges were outstanding." (And if you don't believe that, read How Appealing every day for a while.)