The Settlers of Catan is one of those games I like to own everything for. Because all the expansions for SoC are bolt-on modules (with the exception of Cities and Knights) you can add in one or two of them and make a very different game. Also because Settlers offers scenario expansions (like those found in Traders and Barbarians) and Game Variant expansions like The Event Deck or Harbour Master you can mix and match, altering the way the scenarios play by adding a variant mechanic.
The Helpers of Catan is another bolt-on expansion, that introduces a new mechanic to the game, Helper Cards. Each player begins the game with one helper, assigned by player order. After they use that helper they may return it and take a new one, or flip it over to the other side, allowing them to use that power again on another turn. A helper that has been used twice by the same player must always be returned.
In this way each player has a single special power to help them out, but because you are limited to two uses of it, you can’t abuse your power and players can’t complain that Player A’s power is better than Player B’s.
In my opinion all the powers are interesting and useful. Some are simple, for example William allows you to build a road replacing one of the required resources with any other resource of your choice, while other cards are more powerful and require a sacrifice such as Vincent, who makes you discard a Knight but allows you to build a City for 2 Rock and a Wheat or a Settlement for a Wood and a Clay.
What I like about Helpers is that they help mitigate some of the awful luck swings that can occur in Settlers. For example if you are not getting the resources you need Marianne allows you to take a resource of your choice when you would normally receive none. Or if you are behind in Victory Points Hilde allows you to take a resource of your choice from a player with more VP’s than you.
If another player has stormed away with largest army use Vincent to put your already played knights to better use. If another player takes the spot you were building your roads towards Louis allows you to move your roads to another spot on the board.
So, The Helpers of Catan is about giving players more options and ways out of bad situations. They also help speed up the game, once you know the cards (which doesn't take long). In my opinion, this is a great expansion and its one I would use almost every time I play the base game because, in a very small, uncomplicated way, it helps address many of the problems that can occur. I would even go as far as to recommend teaching new players using Helpers, that’s how much I like this expansion.
I haven't played any scenarios using this expansion yet, so I can’t speak as to whether it would unbalance any of them but with the base game it works great and Lin is the greatest Helper of all because she fixes the “You put the Robber on my land argument”