The Green Cities expansion pack contains a lot of nice buildings and a very welcome change to the traffic system, but it doesn't introduce anything to necessitate what it offers. That being said, it's being sold at a reasonable price, and the changes it offers are fun to tinker with in their own right. And by not taking any risks with altering the pollution mechanics, you can feel free to boot up any old city you have without everyone becoming sick and the air filling up with vaporized sludge. Plus, it's kind of awesome to have a new library of slick buildings and tourist attractions (the climate research station being a personal favorite). It does what it promises, and I will say that if you see it on sale, you really might as well pick it up.