So, I got my first taste of the Count of Carcassonne today in a couple of games with the wife. In the first game we didn't really comprehend it, and failed to use any of the new game mechanics, in the second I forced myself to go against my instinct to use the new mechanics just to see how it worked.
The new tiles and wooden piece are just as attractive as the original pieces, and the new mechanics give you a method of treating your limited number of followers as reserves, placing them in the city of Carcassonne where you can use them to help you claim cities, farms, roads and cloisters as they are completed. You can choose which to claim as they are scored, rather then when you place the tile which reduces the speculative nature of placing followers. In addition, the fact that you can only place a follower in the city when you have given your opponent points means that you change the way you interact with the other players which also makes things more interesting.
The main downside is that it definitely increases the complexity of Carcassonne, and its fixed starting position (not one but twelve tiles!), reduces the variation between games.
I'll have to play it a bunch more times before I can firmly make a call on it, but the early impressions are it'll be nice to play once in a while, but I think I'll still prefer the basic game most of the time.
In summary, the new mechanics noticably change the play of Carcassonne, it becomes a totally different game, and while this will increase the longevity of the game it definitely makes it more complicated. Carcassonne has always seemed a game you won't bore of, it plays quickly (30-45mins), it's very simple and yet there is a great depth of strategy involved and you usually feel that games are won or lost by your moves rather than your luck. Is the Count worth it? Well it only cost £3, so probably yes, but it doesn't significantly improve the game - just changes it.