Just spent a lovely afternoon playing Settlers of Catan with the twins. The age on the box says 10 and up, but they (at 8) seemed to do OK. I think it helped dust off some simple math skills that have atrophied over the summer.
Probably the best part of the time for them was the bartering. They loved trying to trade resources. Also, the fact that their older sisters are going to be jealous that they didn't get to learn to play first. A little sibling rivalry is icing on the cake.
Overall I can see why this game is popular. I can also see why buying the add on pack could make the game go longer. We spent a good 3.5 hours playing, although we had a learning curve to get through. We had removed all of the pieces from the holders a previous evening (which took some time), and our external ring of ocean is slightly wavy, so the hex pieces don't stay locked in. A good playing surface would make things a lot more stable.