As of 2012, there are 11 metro lines , 278 stations and over 434 kilometres of tracks in operation the longest network in the world.
The Shanghai Metro delivered 2.101 billion rides in 2011,the fifth busiest in the world. The system continues to grow, with new lines and extensions of old lines currently under construction.
Shenzhen Metro currently has 5 lines, 137 stations, and 178.44 km of total trackage in operation, and daily ridership is 2.07 million.
Three new lines under construction.
I actually got to travel by metro during my brief stay at Nanjing about a week ago. The facility was pretty clean and modern. Subway trains come with TV so nobody is bored during the short trip. The trains themselves are not as crowded as I anticipated but a lot of people use the train.
Ticket is purchased through vending machines but if you are a long term resident you may purchase RFID equipped cards.
Another benefit of the Nanjing subway is that it provides an underground shortcut across streets and avenues. This negates the need to cross such streets above ground, which could be an adventure in and of itself due to the blatant disregard for traffic rules by many Chinese drivers.