His definition of the doctrine sounds acceptably Lutheran to me (the not-even-amateur theologian):
Put negatively, the significance of the sacramental efficacy ex opere operato is that the bestowal of grace is not dependent upon the sanctity of the minister, nor does the faith of the recipient put any obligation on grace; Christ remains free, sovereign, and independent with regard to any human merit whatsoever. Put positively, ex opere operato efficacy means that this act is Christ's act. ... In the Church's ritual symbolic act, not only are Christ's prayer and worship really present in visible and sacramental form, but really present also is the infallible response to this prayer, the effective bestowal of grace.Obviously if you haven't read the book you can't comment specifically on it. But to any readers who are familiar with the doctrine: Is Schillebeeckx definition adequate? If so, were the Reformer's objections to ex opere operato the result of a misunderstanding?
UPDATE: The Catechism of the Catholic Church seems to say the same thing:
IV. THE SACRAMENTS OF SALVATIONI suppose it all depends on what you load into the opening words, "celebrated worthily in faith".
1127 Celebrated worthily in faith, the sacraments confer the grace that they signify. They are efficacious because in them Christ himself is at work: it is he who baptizes, he who acts in his sacraments in order to communicate the grace that each sacrament signifies. The Father always hears the prayer of his Son's Church which, in the epiclesis of each sacrament, expresses her faith in the power of the Spirit. As fire transforms into itself everything it touches, so the Holy Spirit transforms into the divine life whatever is subjected to his power.
1128 This is the meaning of the Church's affirmation that the sacraments act ex opere operato (literally: "by the very fact of the action's being performed"), i.e., by virtue of the saving work of Christ, accomplished once for all. It follows that "the sacrament is not wrought by the righteousness of either the celebrant or the recipient, but by the power of God." From the moment that a sacrament is celebrated in accordance with the intention of the Church, the power of Christ and his Spirit acts in and through it, independently of the personal holiness of the minister. Nevertheless, the fruits of the sacraments also depend on the disposition of the one who receives them.
1129 The Church affirms that for believers the sacraments of the New Covenant are necessary for salvation. "Sacramental grace" is the grace of the Holy Spirit, given by Christ and proper to each sacrament. The Spirit heals and transforms those who receive him by conforming them to the Son of God. The fruit of the sacramental life is that the Spirit of adoption makes the faithful partakers in the divine nature by uniting them in a living union with the only Son, the Savior.
UPDATE 2: Schillebeeckx later distinguishes between fruitful and unfruitful sacraments:
The sacraments are signs of Christ's redemptive act in its actual grasp of a particular individual. For this reason, even when on account of the recipient's interior dispositions a sacrament remains (probably only for the time being) fruitless, every valid sacrament achieves a certain fruitful effect. It cannot be an empty sign, for even in such a case it is still a sacramental prayer of Christ and his Church for the person receiving it. And precisely on those grounds a sacrament can, as it is said, "revive." If, however, the personal power of supplication of the recipient is joined with the power of the ritual supplication of Christ and his Church, so that the outward sign which the recipient makes is not a fiction with regard to his inward dispositions, then the outward sign by that very fact becomes an effective bestowal of grace, and in consequence its full significance is also realized.So personal faith is not nothing - but it does not affect the power of the sacrament. Even a sacrament received without personal faith "achieves a certain fruitful effect" and can "revive". Still not sure what to make of this but this distinction clarifies it somewhat.