First of all let it be said that I like transit games, so probably I was going to like Cities in Motion no matter what. Let's face it, there hasn't been much produced in the genre in a very long time.
The Good: 1. As noted above, it broke a far too long dry spell in the genre.
2. The game features a very challenging start. While this may be frustrating to some people, I think half the fun of the game is that initial financial struggle.
3. The game has a great modding community, and with that comes great potential. It only ships with four maps, which can get dull after a while, but there are already plenty of community-created maps available (though the largest ones are still in development. Since the maps take a VERY long time to create, the CiM really hasn't been out for long enough that the biggest maps are going to be ready in any sort of complete state). Beyond just maps, there has also been some extremely innovative modding work done on stations, which leads me to my next point:
4. The game is very moddable. By this I do not mean it is easy to mod. It is not. However, for those scripters amongst us, there seems to be very little that is totally hard coded, and it does not yet appear clear exactly where that limit exists.
5. Depending on the year of the game your strategy must change. What will work in 1920 (the game's earliest year) will work in 1980, but won't by any means be the most effective solution. What will work in 2000 won't work in 1940 at all.
1. The game only ships with four maps.
2. There are only 30 vehicles in the base game, and that's spread out over a 100 year timeframe. This can be amended by purchasing the several DLC's that are available, but this doesn't seem to be something that modders have really gotten into thus far, so you will have to pay for new vehicles.
3. The background music gets a bit dull. CiM is a slow-paced game, which in itself is not a problem, but means it really needs good soundtrack. However this is easily fixed by simply disabling the ingame music and running pandora/itunes/spotify/what have you in the background.
All in all, CiM is one of the best transit games I've ever played, and I can only see it getting better in the future.