I'm pleasantly surprised. After playing the demo I suspected that it was going to be a cheap add-on sold at full price. While I must say that it's too expensive at full price the game itself is a lot of fun.
Most stuff about the game has already been talked about here. I just want to add that the campaign has a story line now and there are little 'press' quotes at the start and the end of each missions that give it a lot of flavor. There are cut scenes now too.
I'm only at mission 8 but I think that there are some overpowered features in the game. First, importing is really great. All resources you could ever need you can import now as long as you have a positive balance. Each freighter only brings 500 units at a time. I think this is a little too strong. Now you can start every industry even on desolate islands. I guess the 500 units limit was introduced to keep you from building huge industries based on imported goods, but you can build more docks to attract more freighters to solve that problem. Another overpowering feature, imo, is the nuclear powerplant. Fully operating it will generate over 1000 MW, that's enough to operate everything for a most islands. Granted, you will have to pay to import your uranium but it's a small price to pay to have a gigantic power production with very little land usage. Also, the air conditioning is pretty strong too, if you combine it with your firestation on cat rescue you can get your tenements to over 60 quality. Which is a lot, I guess they tried to solve the annoyance of home building that was T3 (tenements suck, apartments hold very few people) but I'd rather they introduced some skyscraper homes or something than have 60 quality tenements.
But overall I'm very happy with the game. I've played it for about 10 hours now and I'm at mission 8 of 20.