I traveled to Shanghai, China recently in March 2017. My phone is Samsung S5 (from AT&T) and I was able to connect to China Unicom 4G LTE. Ran a speed test, it was capped at around 0.26 mbps. One weird thing though, I didn't get the welcome SMS. So, I called up T-Mobile customer service (free call) and they told me I am covered, no worries. After a few days, my phone out of the blue decided to connect to China Mobile network, connecting at EDGE speed. Really slow. However, now I received the "Welcome to China" SMS. Due to the speed, I decided to manually select network provider and managed to switch it back to Unicom's 4G LTE signal. It worked. The billing turned out OK with China Unicom (it was categorized as Roaming in my bill).

The speed is decent enough to navigate, SMS, Whatsapp/WeChat (SMS, MMS and voice call). Surfing websites are painfully slow, just need to be really patience.