From Bill Bryson's book "In a Sunburned Country":
Nothing so daringly inclined and top-heavy had ever been built before and no one was sure that it could be. In retrospect, the haste with which the project was begun was probably its salvation. One of the lead engineers later noted that if anyone had realized at the outset how nearly impossible a challenge it would be, it would never have gotten the go-ahead. Just working out the principles necessary to build the roof took five years -- the whole project had been intended to last no more than six -- and construction in the end dragged on for almost a decade and a half. The final cost came in at a weighty A$102 million, fourteen times the original estimate.
Utzon, interestingly, has never seen his prized creation. He left the project in 1966 in a dispute over rising costs and has never been back. He also never designed anything remotely as celebrated.