I've played plenty of Settlers and Cities and Knights is a far better expansion than Seafarers, in my opinion. I will elaborate, but if you'd like to give it a shot I think you can download and run Cities3D:
http://www.settlersonline.net/It worked for my group of friends years ago. From playing online, I decided that Seafarers wasn't really worth purchasing. But C+K, definitely.
Cities and Knights does not dilute politics, but it rewards strategy significantly more than the original. It actually adds a hint of co-op play as well.
There are two major changes. The first change is the introduction of Barbarians, the second is the introduction of Commodities and a bunch of stuff that goes along with that.
With C+K, you roll three dice instead of two. There is a red numbered die, a regular die, and an event die. The event die has 3 Black Ships. The other 3 faces have yellow, blue and green castles (or city gates or something, I don't quite remember).
Barbarians sit on the board along a track. Whenever a Black Ship is rolled, the Barbarians advance one space on the track. When they reach Catan, they attack. This is the co-op aspect -- all players together "must" defend against the Barbarians. This is done by building and activating Knights before the Barbarians arrive. If the collective number of active knights matches or exceeds the collective number of cities built, the Barbarians are repelled and the player who had the most Knights wins a Victory Point. If the Barbarians win, whoever contributed the least Knights must demolish one of their Cities (they downgrade it to a Settlement).
This change in the game adds a constant pressure and also opens up a lot of strategic area. I've won many games in a landslide simply because I activated a Knight and my friends forgot to do so, losing their initial Cities to the first Barbarian attack. (For this reason, you should really warn the other players of the importance of building and activating a Knight, or else they will be behind the entire game and probably not have much fun).
Commodities are a new type of resource. They are Cloth, Coin and Paper (for Commerce, Politics and Science), and they are produced by Cities on Sheep, Ore and Wood hexes. A City on a Sheep hex will produce 1 Cloth and 1 Sheep instead of the 2 Sheep in the original game.
Commodities are used to purchase City Improvements. This is where the red die comes into place. Suppose you have the first Science City Improvement. Now, if the event die rolls green and the red die is 1 or 2, you get a Science progress card (similar to the development cards in the original). As you buy more City Improvements, you will get cards for higher numbers. The third improvement also nets you a special power (e.g. Commerce will grant you the ability to train Commodities at 2:1). If you are the first to reach the fourth/fifth level, you can convert a City to a Metropolis (worth more VP, cannot be demolished by Barbarians).
I am trying to strike a balance between summary and detail and it is difficult. But suffice it to say that C+K is an excellent expansion. Politics remain, but you also need to be more careful in getting the right resources and building the right things.
However, I would NOT recommend the expansion if the other players would be daunted by complexity. It adds a lot.
