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FunFunFun
Anyone who has played the original game will love this expansion. It involves more depth to the original as you have to develop your knights in order to succeed. Playing time varies with the amount of people. Very fun with 5 or 6 with the expansion. I would recommend this game to anyone that loves the original or that likes to play board games in general.
P.S.
The only thing I would caution is if you have any animals, the pieces tend to break. This is a personal preference, just thought I would throw that out.

A New Degree of Complexity to Settlers
While the Seafarers expansion added some new wrinkles to Settlers, the game still felt like 90% of the original. Cities and Knights, however, transforms Settlers into a much more complex, longer, sophisticated game. Our group usually finishes a game of Settlers or Seafarers in about 1 hour with lots of talking, laughing, and hooting. But our first game of Cities/Knights took almost 4. I'm sure that would decrease with experience, but with so much to keep track of and plan for, we found that we weren't interacting with each other as much. Ultimately, we didn't enjoy it as much. But if you have mastered Settlers and Seafarers and want to ramp up the strategy, then this is worthwhile.

Interesting
It makes the Catan game a totally different game. More complicated and more interesting. The only problem would be that it makes the game time much longer than the original pack, and the rules are much harder to understand. Suitable for senior players of Catan.

Adds complexity but in a good way...
I got this expansion along with the original game and played it after playing a couple games with the original rules. The basic changes to the original game include:

1) Instead of the development deck, you now have three progress decks (sciences, trade, and politics) which offer a broader selection of usable cards than the development deck and it's 90% knights and 10% events/actions. These decks correspond to three possible areas of improvement (see next item) and have some interesting effects (everything from allowing you to take cards from another player to pulling resources for free).

2) Cities can now be improved. You get a set of flip cards that you flip as you purchase city improvements. There are two benefits to city improvements: a) when you achieve the 3rd level of improvements you gain some bonus like the ability to trade 2:1 on any resource, and b) each improvement increases the changes you'll get to pull from one of the three progress decks.

3) The addition of an event die that you roll along with the standard 2d6. The event die will either move the barbarians closer (50% chance) or trigger a chance to pull from the progress decks (16% chance). As mentioned, city improvements increase your chances of scoring a card when one of the progress areas are rolled (i.e. if you get a 1 or 2 on the red die and a blue icon on the event die and you have the first city improvement in the science area, you can draw a card).

4) barbarians have been added on top of the robber that still plays as it does in the original game; the barbarians show up after the barbarian icon shows up on the event die (which is more often than not). When the barbarians reach Catan and if there aren't enough knights in play to protect Catan, then the weakest player (in terms of knights) who has a city will lose that city (it gets downgraded to a settlement) as it gets razed by the barbarians.

5) knights are now pieces in play rather than a drawn card; they can bump other knights and the roober and play a crucial role in dealing with barbarians: if the number of knights who are active exceeds the number of barbarians (= number of cities in Catan), then the players win and the player that contributed the most will receive a special Defender of Catan card (ties result in progress card draws) which gives you a victory point.

6) Lastly, to make things interesting, there are commodities now, coin, paper, and cloth (which correspond to iron, wood, and wool resources) which are primarily used to buy city improvements. You get them if you have cities (i.e. instead of getting 2 iron if you have a city next to an iron spot, you get 1 iron and 1 coin).

Yes, it's definitely more complicated than the original rules but it offers a choice for anyone who wants that complexity (me!). It makes the game deeper and in some respects fixes issues I had with the previous game (like the knights being way too easy to pull up off the development deck given their numbers).

Once you get used to the rules (one or two games will usually do it), things move along and tides can turn pretty quickly (like when you were unable to active your knights before the barbarians came and you end up losing a city...or when Catan still wins but you just handed your opponent a Defender of Catan card which secured another victory point).

All in all, if you liked the original game and are either bored with the simpler rules or want to mix up the game a bit more, then I highly recommend this expansion. The added rules and expanded progress cards and city improvements really evolve the game in a good way and bring out the best of this game.

Oh, and note that you need to use this with the 4th edition (Amazon made sure to label these with that big "New 4th Edition!!!!" tag...). I never had the original versions so it didn't matter to me but some reviewers seemed to have an issue with getting the wrong edition so...

Family Fun!
This is a wonderful, highly addictive family board game. We got the original Settlers for Christmas and loved it so in January we saw this at the toy store and thought we would try it. Well, 3 months later my wife and 10 year old daughter and I are still playing it almost every night. It is a little more challening than "Settlers" and has even more variations and strategy so I would not recommend it for children less than 10. You have to have the original "Settlers of Catan to play. It takes about 20 minutes to learn to play and games typically take 1 to 2 hours. There are all sorts of different strategies to use and because the board varies each time you play no two games are the same. There is some cutthroat potential in the game so if your family is prone to violence you might try something else. On the other hand if you are looking for a fun, challenging game that will get your children and spouse off the computer and television to spend some time together this is highly recommended.