I first heard of this book when it was being promoted (read flogged) at youth group. The thing that sort of put me off the book was that at the time the title did not seem to be all the connected with the contents of the book. In fact, the title probably more suits a book like Francis Schaeffer's trilogy, which in sort challenges our modern mind set and tries to encourage us to return to a more spiritual and interventionist reality rather than our mechanical and disconnected modern society. However, when I think about it I can see where Carson is getting at in that prayer is the essential element to a true spiritual reformation.
The problem with Carson though is that he tends to write in a way that is not really all that accessible. Don Carson is a very well known (and popular) evangelical theologian, however I have noted that he tends to use complex words and discusses complex theological topics as if we are already versed in these concepts. As such, it is not the type of book for new Christians, nor is it a book to those who are not academically inclined. Christianity (as with any subject) is full of jargon, and using the jargon to attempt to explain such complex theological concepts to those who are not familiar with the jargon will inevitably result in failure.
The other issue is not so much with the book itself but rather how my former church responded to the book. Basically they thought that the content of the book was so good that it pretty much formed the basis of all of their talks on the subject of prayer. I remember once at youth group the leader decided to lead a number of talks on prayer, and in doing so, pretty much recited verbatim from the book so that when I went to read the book myself I suddenly realised that I had heard it all before.
Now, Carson uses Paul's prayers as a model on how we should pray, and granted, while he is using that for the purpose of this book we must remember a few things (particularly since my former church did not take this into account at this time):
1) These prayers are written prayers, and while we can use them as an idea as to how Paul would pray, we cannot expect that this was the only way that he prayed, or that this was how he prayed when he vocalised his prayers. Also, being written prayers, they are enmeshed into the works of his letters, and while I don't actually see pastors use this technique in their sermons (they tend to open and close in prayer, but rarely, if ever, do they enmesh a prayer into the sermon), it still requires some work to pull them out, and we also can't pray them verbatim.
2) Paul's prayers aren't the only prayers in the Bible. While Paul's prayers are good and can work as a model, for quite a while my church seemed to forget that there is a much, much, better model prayer in the Bible, and that is the Lord's Prayer. First of all, when he was asked how one should pray, Jesus responded with the Lord's Prayer. Secondly, it is a vocal prayer, and many, many, churches will recite this prayer during their services. Finally, it is actually a very good model prayer, and for those of us who do not know how to pray, or struggle with prayer, the Lord's Prayer is a brilliant model to turn to.
As such, while this book may be a good book, and also have some use, there are actually much better books out there. As such I would only recommend it to those long term Christians who are comfortable with academic works.