I picked this up to fill out a 10 for $X at a second hand bookstore, and when I offered to lead a few lessons on the church for our Bible study I decided it was time to read it (among other things, of course). Kung is a pretty famous figure in the Catholic church--and a controversial one.
The general approach he uses assumes that the church, being made up of humans, is often in trouble and needs to get back to its roots in every age--and so he emphasizes Scripture more than Tradition. He systematically addresses the various aspects of the nature of the church. He is especially annoyed with those who focus on the true church as the church at the end of time--it isn't a very useful model to apply to the church one finds here and now on the corner.
When he gets to the section on church organization he brings in a lot of detailed history. If we stipulate that his description of the history is correct (I'm in no position to say), then it is quite plain that the hierarchical model of church structure was not only not universal in the early church, it existed only sketchily in places and not at all in others (Corinth, obviously--Paul never addresses himself to church leaders, even in matters of church discipline he assumes that the entire church has a say). Only gradually does one find a single supervisor/bishop instead of a group. He treads more softly with the pope, but the weakness of the claim of primacy, not to mention dominion, is quite clear; and Kung offers advice on how a pope should serve and which titles ought to be abandoned (pontifex maximus was a Roman pagan priest's title, but "servant of the servants of God" conforms with Christian teaching).
I'm Protestant, and he's not goring my ox. Even claims that Scripture does not exclude the possibility of a pope to head the church do not ruffle my feathers--the Bible plainly neither requires nor prohibits a pope. But I can see why he got a lot of Catholics angry.
I gather that English is not his first language. Scholarly prose in translation is thick going, and he has a little tendency towards repetition for emphasis. Quite a few interesting points are skipped over, as he refers you to some other book he wrote or relies on.
He also gives rather more credence than seems reasonable to "higher critical" analyses of the Bible that attempt to dissect which bits came from which era within a single gospel. I've never heard of anyone attempting a double-blind study of the "higher critical" method (obviously using a different book), and absent such verification I class it with homeopathy. His handling of it doesn't injure his orthodoxy, however.
I read the book because I wanted to learn, and was willing to accept the rough sledding of scholarly prose as the price of knowledge. It is an expert's book in a field in which I am not an expert. If you're interested in the church you may find it useful.