This week I've a couple of games of Carcassonne using the "Inns and Cathedrals" expansion, and it's been a good experience.
Basically it includes a number of new tiles.
A couple of "Cathedral" tiles, which are entirely city with no green edges. This adds a really fun effect. When added to a city that city becomes totally valueless unless the city is completed. If completed the city is worth extra points.
Likewise the other tiles include road tiles but they roads have inns next to the road. Like the Cathedral if completed the road is worth extra, but if not complete the road is worthless at the end of the game.
Both of these features cause a few things to happen, especially in the end game. If you draw a Cathedral tile or an Inn tile and an opponent has a big feature going you can take a gamble and play this poisoned chalice into their feature spoiling it for the end game score.
Of course you can likewise take the tile and try to make the most of it earlier in the game by scoring extra points.
As well as these great score effecting tiles, it also includes a number of extra tiles that are new, and will cause the tile-counters to have to recalculate. These are not repeats of existing tile types but whole new shapes that give you new great options.
This expansion is a worth while purchase for a Carcassonne fan. It adds new scoring options without either unbalancing the game or breaking the existing stratergies.