I was brought into Catan by a friend and I bought the Cities and Knights Expansion so that we may pump up the excitement.

The original Settlers of Catan is plenty of fun, but with Cities & Knights, players take on much more specialized roles. Game play is more complex -- but if you can play regular Catan in your sleep, you'll find Cities & Knights intuitive and raucous. Beginners should definitely master plain Catan first.

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.

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.

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.