The Settlers of Catan

Settlers of Catan is one of those games that I have heard lots of good things about, but have yet to play myself. How often is it that you see several expansion sets and spin-offs of a board game? There's go to be something good going on with this game. If anyone has firsthand experience make a comment. By the way, it pretty consistently gets 4-5 star reviews at Amazon.

The Settlers of Catan - New 4th Edition at amazon.com
The Settlers of Catan 5-6 Player Extension - New 4th Edition at amazon.com
Mayfair The Settlers of Catan Board Game at amazon.com
Cities & Knights of Catan: 5-6 Player Expansion at amazon.com
The Settlers of Catan at ebay.com