I've played with five and six players via the expansion, and it does as good a job as it could, but the truth is that four-player Catan is the way to go.
By the way, and this is a quick aside, Settlers of Catan should be played with four players. Fewer players throws off the balance and flow a little, and two is barely tenable. I've played with five and six players via the expansion, and it does as good a job as it could, but the truth is that four-player Catan is the way to go.