There is some complexity in its simplicity. But lacks something...
Whats the game about?
Transportation. You make your own little transport company that apparently always has a monopoly on all the city's means of transportation and you build the transporting infrastructure for the CIMcivilians.
How does it work?
Basically, there is a simple and efficient process to progress in the game. Scan the city using some helpful info tools. Decide how to get from A to B (bus, tram, metro, waterway, helicopter). Build the stops and infrastructure to get there. Make a line. Attach transport vehicles to that line. And then repeat the process.
So whats great in it?
Integration and progression. The city is a living entity... like the world its a part of. It suffers economic booms, downfalls, modernization, unemployment, etc. You get requests from VIPs or from associations for public transportation. You rarely haven't got anything to do... but sometimes, you can't do it because you don't have the money to invest in it. The difficulty on the game is also very good. Just in the first mission of the campaign (there is a campaign and sandbox games-5 so far-) took me 5-7 restarts until I passed it. So its challenging. Also, the game evolves. You can play from the 1920's to the 2020's in terms of what means of transport to use. And as it progresses, you get new toys to play with that, if you sandbox for 30 years, you will have to change and modify you lines and your vehicles to suit the more modern times. Its pretty cool.
Because there are some bad things in it:
Banks are still loan sharks and crooks as they are in real life. Integrating and correlating between the tools made available to you needs to be reworked on. Your choices of doing stuff are very limited. You cannot directly influence city evolution and expansion. You don't get to make research in transportation or invest in it, you just get better vehicles as time passes. You don't get to support political candidates. You don't even get to transport goods... just people. So you are limited... very limited.
Still, this limitation has its usefulness... in that it allows you to focus solely on what the game is about without any off-topic distractions. In that, it accomplishes all of its tasks... it offers you a very good transport simulation game. But for a long-time gamer like myself, it just isn't enough.