I agree with Skogs with regard to Greatest Hits albums simply being another part of an artist's contract nowadays. I really despise it when you look at a track list for a greatest hits album and it is clear as day that the band had no input over what tracks were going on it.
I think some greatest hits albums are good though, notably ones which have come out after a long spell, like The Very Best of The Eagles and U2's Best of 1980-1990.