I found this article about Bob DeWaay's encounter with Purpose-Driven ideology, and his meeting with Rick Warren challenging and thought-provoking. I have not previously read anything by DeWaay, but I hope to follow up some of the links he has given in this article.
One of his main points is that Warren correctly teaches some of the gospel message, but also urges people to follow some of his ideas that are not from Scripture, with the same urgency he gives to teaching parts of the Bible's message.
He says that DeWaay inappropriately uses the Scriptures and addresses non-Christians with teaching that is meant specifically for Christians.
He also says that Warren comes across as a liberal Christian, but that when you dig deeper you find he has a conservative basis, buried underneath the facade. But he doesn't teach much of what he says he believes.