If you are coming to China for travel, work or living — and you are a self respecting citizen of our modern world — then you will need a China SIM card in order to stay connected to the web through your mobile phone and to make calls (or SMS text messages, but I don’t know anybody who still uses SMS now in the days of Facebook messenger, What's app, LINE, We Chat, and Kakao Talk). You will also probably want to get a VPN so you can connect to Facebook, Google, and YouTube.
In most Asian countries that I have travelled to it is usually a breeze to get a pre-paid SIM card. They are cheap, easy to recharge credits, and you can get connected in virtually a couple minutes. Just make sure that your phone is already unlocked, and you will have no trouble using a new SIM.
China mobile networks operates much in the same way, however there are a few wrinkles that you should be aware of — so here is a guide to help you figure out which SIM card to get when you come to China.
The largest mobile carrier with about 70% market share. China Mobile shopfronts are EVERYWHERE in China. So even if you for some unknown reason find yourself out in rural-nowhere China, you might not be able to find a real toilet, but at least you will still be able to get a SIM card there.
Coverage: Very good. Virtually everywhere in China.
The 2nd largest mobile carrier in China, and is the only carrier that has 3G that will work with phones from outside China.
Coverage: China Unicom coverage is not as extensive as China Mobile, but is still very good. It covers nearly 90% of the population, so chances are you will be covered wherever you are.
China Telecom uses a CDMA network which is incompatible virtually all phones purchased outside China. If you are thinking of purchasing a phone that only works in China (why you would do that, I have no idea), then I guess you could look at China Telecom. Otherwise, pass on it.