I can tell you what I know, which isn't nearly enough.
There have been Catholics in China since the time of Mateo Ricci, in fact some Catholics visited China before his time.
The church was dominated by missioners up until the civil war, and the Catholic church was negligent in developing a native episcopate until it was almost too late. Thus it came to be seen as a foreign religion in a land that was to quickly become hostile to all religion.
The Communists drafted legislation that any religious organization had to be run by natives. This was not so bad for the Protestants, who quickly adapted. The missions run by Protestants were quickly turned over to their native lieutenants. It was not so simple for the Catholics.
The thing is, the Papacy claims universal jurisdiction, and the right to name all bishops (it was not always this way, but in the modern church that's how it is). This means that the Catholic church is in theory controlled from outside the country. That is against the law in China.
As a result, the Catholic church in China has been somewhat abandoned by the rest of the church over this principle. The evangelizing efforts of the native Catholic church have been crippled and the native Protestants have stepped into the vacuum.
Yes, their bishops do have Apostolic succession, and yes their sacraments are valid. Some of these bishops of the 'official' church actually have been approved by either Pope John Paul II or Pope Benedict XVI, and so are in communion with the Pope. These men are in communion with the Pope and also with bishops the Pope did not approve, so the church is not in schism exactly, it occupies a gray area neither here nor there.
I can assure you, Chinese Catholics are as faithful as any Catholics anywhere, possibly more so. They are devout and orthodox in their faith and adore the Pope, whose picture can be seen in any rectory. They are strongly devoted Marianists too.
But, they have this political problem not of their choosing.
I don't think the underground church is anywhere near as large as people think it is, and frankly from what I can tell most of the 'underground' sympathizers regularly attend Mass in the official church too, so they might be counted twice.
Hong Kong is a part of China, and the government leaves the Catholic church there alone as part of it's 'one nation - two systems' policy. (The government even lets the Falun Gong make public displays and demonstrations in Hong Kong.) Same for Macau.
Bibles are easy to purchase (in fact many Bibles sold in the USA are printed in China). But when I lived there it was very hard to find a Catholic version of the Bible, the church (which owns the translation copyright) lacks sufficient funds to publish, and so many Catholics have to make do with the Union Bible (Protestant).
In spite of all the rhetoric, priests, seminarians and nuns from the 'official' church come to study and work in Roman Catholic dioceses, even in the USA. Catholic immigrants from China are very conservative and devout and are accepted readily.