There's no charge for it. You can register on the website in advance or on the plane. You do need a vpn if you want to use sights blocked in China.
It works very intermittently and is very slow to the point it is mostly unusable. Last few flights I gave up on even trying to use it. I think it used to be better a couple of years ago even though it was never great. Maybe since they allow people to use their phones now more people are using it.