Settlers of Catan is a VERY fu game for non-gamers and gamers alike (I am an old-school wargamer. Probably the best introductory game to the new German game boom. The only thing I found lacking was alot of variance in repeated plays, despite the map changes with different hex placement.
I have Knights and Castles and found that it took away alot of the GOOD from Settlers (trading, light strategy, and back-stabbing: racing roads to a new settlement after trading and breaking roads). It adds unwanted complexity and lowers player interaction, in short -makes the game LESS fun.
Seafarers on the other hand, adds tremendously to the game. My gaming group absolutely loves Settlers (Im the one that suggests new games). Seafarers adds the same kind of light strategy, trading, double-dealing, but manages to keep ALL of these aspects of the original AND still give you more options!
An expansion NO Settlers of Catan player should be without, this one completes the (original) Settlers of Catan experience.
Ship Ahoy!