That's not to deny that theologians of Küng's stripe and generation have performed a signal service to the Church. A peritus at Vatican II along with Ratzinger, whom he had headhunted from the University of Münster's theological faculty for that of Tübingen a few years earlier, Küng is probably the best-known of the "progressive" theologians who dominated, if not the Council's deliberations themselves, at least its interpretation in the two or three decades that followed. Books of Küng's such as The Doctrine of Justification; Infallible? An Inquiry; and On Being a Christian, though not of the very first rank, were trenchant enough to force the orthodox to rethink their approaches and get creative. I rather doubt that authentically Catholic theology today would be anything more than a relic if it hadn't been for troublemakers like Küng and others. Though Ratzinger was right back in 1979 to help engineer Küng's de-certification as a Catholic theologian, he and John Paul the Great were also right not to excommunicate him.
Küng, along with the others of his ilk, are largely irrelevant dinosaurs today, but they did play an interesting role, it seems. If I may play the "Darwinian" game just one more time here, Küng's ideas were like a virus infecting the Church, and Mike is right that genuinely Catholic theology had to either sink or swim. That it not only swam, but drowned out its dissident opposition, should reassure us all that orthodoxy is stronger now than ever, having survived a dangerous threat and adapted to the conditions that gave rise to that threat. When the next wave of dissent comes along--and it is bound to happen (we're only human, after all)--we may at least take comfort in the fact that orthodoxy grows ever better adapted to such threats, helping to insure that the gates of hell will not prevail.