It was for me great fun to play this game in the begin - However there is a super enoying bug that will happen eventually in any city with metro trains and renders the whole system unplayable.
Usually in the late stage of the game there will be always an important metro station where trains rediculously will just not leave before they are completely full (which takes very long because they are waiting very long for single people who are just entering the station)!!! This causes heavy jams in front of the stations where trains que up infinetly. People on subsequent stops can hardly enter the train thus get angry. Also you can not add more trains to the line because they will only add to the que in front of this one faulty station
This is messing with the whole (!) metro nework and there is absolutly nothing what you can do about it ...
The bug is known but the developers sadly don't care about it.
Also beware of high CPU Loads and memory leaks. After some time the Game will slow down as hell.
The missions are mostly funny, but instead of encouraging you to build a reasonable network to guide you it rather forces you to build non-profitable lines.
There is in general not much assistance or feedback to understand why your system is working or not as well as the statistics are not so useful.
The mod system is great - very active community.
bottom line: In the beginn it's a good fun to play this game but it lacks long term motivation because of enoying bugs and lack of reasonable feedback.