As has been pointed out, Warren's tome is a series of meditations. If he's able to coax lukewarm believers into thinking about God for forty days, he's accomplished something considerable.
I think some of the theological and biblical purists are just jealous that they didn't think of this approach first.
That is not exactly a biblical truth. Broad is the gate that leads to destruction, narrow is the way... Just "thinking" about God does nothing. Without repentance, there is no forgiveness of sin. Thinking about God is not accomplishing anything considerable if that is all that is done and it goes no further. But if one biblical expose is the means which God uses to cause one soul to come to repentance, then all of heaven will rejoice and I consider that considerable!
Yet, I know this is PDL is the latest fad and is something that is swallowed by many and there is not much I can do to help people understand that wordly methods do not work on the souls of men but it salvation comes from hearing the Word of God. It has always been, it always will in this age.