I have to say that the previous reviews are about as thorough as I could be, so I only want to add a few general cents here about the text.
First, his desire to have the Congregationalist/Free Church recognized as legitimate ecumenical partners for dialogue is very welcome. Since they represent one of the fastest growing segments in modern Christianity, it makes sense that they should have a voice. But a common voice would be helpful. Congregationalists are a little like talking to Hydra- who is the voice? Volf offers a defense or vision of his ecclesiology, but in the end I believe it remains just that, "his" defense of "his" ecclesiology. In my own dialogue with "Free Churchers" there always remains the but-we-don't-see-it-that-way factor that is hard to go beyond, when the next one can totally agree. I certainly believe that Volf's ideas will find resonance with many readers/prayers/hopers, but in the end, it still lacks the unifying force that remains in the mainline traditions of Roman Catholicism and Eastern Orthodoxy (although some would argue that Orthodox are not unified in any real sense, but that's another story...) which leads them to lack a real, unified alternative to modern society. In chapter 3, Volf argues that it is the Holy Spirit that actualizes the Church. Of course, but that is also the problem with Free Church theology. Its theological identity is always playing second fiddle to its non-conformist, non-structured ecclesial identity, leading to a least common denominator "denomination". What is most essential is then the question. But since when is the Tradition or scripture minimalist? Volf doesn't help there. Although chapter 6 tries to define what it means to be "catholic" (according to the whole), it really doesn't ring true.
Second, his ecclesiology and trinitarianism tend towards individualism, since he still fails to deal substantially with the Eucharist and Baptism and traditional Trinitarian theology (the heart of traditional ecclesiology). Since his ecclesiology is essentially individualistic (Enlightenment?), it makes sense for him to do this, but it totally misses the point. Even while he claims trinitarian models for his approach, I found them lacking in substance. God is more than three roles, three persons united in love or a common substance. (He follows Moltmann's lead in seeing the three Persons of God as individuals united in self-giving. There is certainly precedent for this in the past, starting with St. Gregory Nazianzus' Christology, but it still doesn't go far enough, or perhaps it goes too far!). What unites "God" is the Father- God proper. The Son and Spirit are not the head. What really defines Christian theology is that God exists not as three individuals with relations to each other, but as three who are relations. There is a monarchical order, even if it is beyond our understanding. And this, too, Volf criticizes in Zizioulas, since faith for Volf is rationalization, not the faith of children. Z argues for a suprarational approach/experience of the Church in the context of liturgical realities, not cognitive reflections. And this is the real heart of the issue. If we are to image the Trinity, our imaging is in what we are. I am not really me unless I am united to you. Perichoresis as Volf and Moltmann use it must move beyond united individuals in freedom (Congregationalist ecclesiology). An individual, or the isolated, self-defined Church, is not a person or church in the traditional theological sense.
This book would have been much more useful had it used the sacraments/mysteries as touchstones of ecclesiolgy.
I would suggest reading Zizioulas' "Being as Communion" or "Eucharist Church and Bishop", McPartlan's "Eucharist Makes the Church" and "Eucharist and Church Fellowship in the First Four Centuries" by Werner Elert for a more comprehensive understanding of these topics that Volf ignores outright.