So, I was at the hobby store yesterday, and I saw a bunch of boxes in different colors with the Settlers logo on them. I was vaguely aware there were expansions for the game, but I didn’t know what any of them contained. I read the backs of the boxes, and they sound complicated but kind of interesting. Some looked like they added a new element to the game, and some just looked like they were throwing in new resources and tiles just for the hell of it. Of course, the boxes don’t say which one are fun and which ones aren’t, so that’s why I’m here.
Tl;dr - which settlers expansions are worth $40 to a poor grad student?
Cities & Knights will change the game completely with a bunch of new game mechanics (knights have a presence on the board, commodities that upgrade your civ along 3 paths, which produce cards along 3 paths, etc.). I can’t even imagine Settlers without it anymore.
Seafarers is useful because playing on the same type of map can get boring, and you can really be creative with how you structure the game board, but I suppose it’s not as essential as Cities & Knights.
I second Cities and Knights. It increases the replayability so much. About the only thing you lose is not knowing how many victory cards your opponent has in his hand, and you gain so much more.
I’m not as big a fan of Seafarers, because the lack of accessible real estate slows down the game a bit.
I don’t recall if Alexander was a purchasable expansion, or only available online. But it introduces bidding for settlement areas into the equation, which I happen to adore (but people refuse to play bidding games with me for that reason :))
Seafarers adds a new use for sheep (used to build boats) and has some (optional) new VP mechanisms (get VP for building on a new island) and optional discovery (hexes are upside-down until someone sails to them) - but still mostly the same game.
Cities & Knights as Windwalker says, makes it a significantly different game. More a gamer game than a family game. Some like it, others thinks it adds too much complexity
There is also Traders & Barbarians. This packages different mini-expansions in one box:
Brian
I’ve been playing Catan and the Seafarer’s expansion for ages. I got Cities & Knights for Christmas last year and it’s still in the box because I have no one to play with 
We have all three.
-
As others have said, Cities and Knights adds a lot of new gameplay. There is a lot more depth and many more strategy variations than the basic game. I think it is really well-balanced; possibly better balanced than the basic game. We tend to play Cities and Knights more than the basic set, but we do still enjoy the vanilla version. If you are thinking about expanding, I agree with others that this is your best bet. You’ll really enjoy it.
-
Seafarers adds some additional rules that aren’t very complicated. You could probably make your own version using pennies for ships and a dime for the pirate. The set comes with a pile of new hexes – including sea hexes. We haven’t played Seafarers that much because it does seem to slow the game down. Our group includes three kids and anything to keep the pace going is welcome.
When you are playing with a lot of sea hexes, the game focus shifts. You need to collect a lot of wood and wool to build ships to traverse the water hexes. In the meantime, you aren’t increasing your incoming resources. Compare that with the equivalent strategy on land: If you are building a long road, you can always plop a new settlement along the way.
This set also includes a number of game scenarios which are cool enough, but I am not a big fan of playing the same scenarios over and over. I prefer starting a standard game with a blank set-up.
- We just got Traders and Barbarians and haven’t even played through it all yet. I haven’t even read the whole rulebook. The set is chock full of various new pieces for the 5 scenarios. There also some new variants that can be applied to any version of Catan.
One variant that I really like are event cards. The resource dice are eliminated and are replaced with cards. Each turn you draw a card and it tells you what you ‘rolled’. The distribution of cards matches the ideal distribution of die rolls so that the effect each number comes up as often as you expect. You add a little bit of randomness by discarding a few of the event cards after each reshuffle, but it more or less tracks an ideal set of rolls. This works well for us since we play with kids. It eliminates the case where one of the kids gets very few resources because of a quirk of the dice.
I noticed I have a bias against game scenarios – whether in board games or computer games. I prefer expansion sets that add new rules that can be applied every time you play. As a result, I favor Cities and Knights. For those groups that like scenarios, they might prefer one of the other sets.
One option to consider is to just create your own expansion rules or scenarios using whatever playing pieces you have laying around. I get the feeling that a lot of the expansion set content was born this way.
Two other things to consider: First, the basic set supports 4 players and requires an additional pack to expand to 6 players. If you didn’t already notice, each expansion set works the same way. If you are already playing with 5 or 6 players, then you’ll need to get an expansion set and its expansion set.
Second, you can buy loose additional hexes and borders separately as well. This allows you to create bigger maps or maps with sea hexes. And it might be a cost-effective way to expand your game without springing for an expansion set.
On the downside, since you now need them both, you no longer have as many people saying “I’ve got wood for sheep!”
Cities and Knights - yes, definitely. Seafarers - don’t bother.
Wool sails! Those’ll be good once they get wet. 
Concurring with some of the other posts: Cities and Knights is a great expansion. Seafarers isn’t bad, but it makes it a substantially different game.
I can’t speak for everyone, but I myself would get a lot of mileage out of “I need sheep for my seamen.”
I love Cities & Knights, but it does make the game more complex (a good thing in my book, but can be bad in some crowds) and also makes the game significantly longer in my experience.
I haven’t played any of the other expansions. I should probably buy them. I love Settlers. Settlers, Dominion, and Puerto Rico are my go-to games.
Conversely to a lot of the thread, my gaming group has a lot of traders and explorers so Seafarers gets regular play while no one bothers with Cities and Knights. Your mileage WILL vary, so I’d read the reasons why people like the different expansion and go from there with attention paid to your gaming groups’ tendencies.
Seafarers is a nice change of pace, and it allows for alternative expansion rather than getting boxed in. Plus, it has a border which means I don’t have to get neurotic when it comes to people bumping the board and shifting the hexes.
I’ve heard some great things about Cities and Knights but don’t have anyone to play with in our area. My wife and I test played it but two players made the game last too long.
If you want to try them out, you can go here and register to play “Xplorers” for free, it’s basically a free online version of SoC. I never tried the cities and knights expansion, but i did enjoy the variations in maps especially with water tiles and boats.
I’ve seen that site before - it seems to be the only place to play this sort of thing online. However, they’ve stopped allowing new members, apparently because there have been some mean people join, and I can say from personal experience that this ban has been in place for at least nearly a year, so I’m not holding my breath for it to open back up.
Cities and Knights: A-
Seafarers: B
5-6 Expansion: C-
That is one thing I will say–the 5-6 expansion is all well and good, but some gamer groups are totally incompatible with it. Like mine, in which we had two or three guys who could take 15 minutes on a single turn, and having both in a game for 6 players made it unbearably stupid.
Being allowed to build during other peoples turns ruins one of the few strategic aspects of the regular game. If you want a 5 or 6 player game I highly recommend the OP plays something else.
Yeah - getting four people in the same room at the same time is really the most challenging part of the game; I really wouldn’t have any use for the 5/6 expansion. Is it just an extra ring of hexes around the island?