If anyone studies Historical Theology, then this is a good place to start for the 20th Century. I must admit from the start that I agree with very little which 20th Century Theologians espouse (I am a confessional, Reformed Protestant who adheres to the Westminster Confession), but that does not stop me from enjoying things written by them and about them.
Grenz and Olson write from a sympathetic, yet critical standpoint. Their main analysis is over the transcendance/immanence tension throughout the history of Christian Philosophy. The 20th century theologians are a reaction to the classical liberalism of the 19th century, but in my opinion, they do not go far enough. The theologians they survey that I enjoy the most are Karl Barth, Karl Rahner and Wolfhart Pannenberg. The ones I dislike the most are Jurgen Moltmann, Paul Tillich and Hans Kung.
I also enjoyed reading about Narrative Theology, too, because I think some of it is similar to Reformed Biblical Theology (see Geerhardus Vos, Herman Ridderbos or Richard Gaffin for an example). Reformed Biblical Theology places its emphasis on the outwork of the Redemptive and the Christotelic nature of scripture, much as Narrative Theology places its emphasis on the developing story or narrative of scripture.
The big issue I have with 20th century theology is its doctrine of God. The big theological move is more toward a panentheism, rather than the distinct creator/creature distinction. They see God's transcendance/immanence in temporal rather than spatial categories. This has led to the questioning and denial of the Immutability, Impassibility and Foreknowledge of God. The logical conclusion, in my mind, is process theology, which holds that God is Di-Polar. His pole of being is finite but his pole of becomming is infinite.
My biggest problem with the book itself is that the authors are very open to this possiblity, but personally I am not, and see it as nothing but Hegelianism. Other than this caveat, however, it was a good introduction to the subject and I would recommend it to anyone studying the theological landscape of the 20th Century.