John Piper is especially worth believing on the Doctrine of Penal Substitution. He wrote the Foreword to Pierced for our Transgressions. He ends:If I were going to choose who to believe I would probably lean towards the TGC guys (I have enjoyed D.A. Carson and Tim Keller's words for some time now, and John Piper is one of my favorites...so please don't take offence).
".........Every section of this book yields another reason to thank God for the labours of the authors and for IVP in Britain. I pray that the Lord will give the book success in the defense and honour of God, and that Jesus Christ will be treasured all the more fully when He is seen more fully to be Pierced for our Transgressions."
Don Carson has also written a commendation for the book (as has Mark Dever).. Carson seemed very strong on P.S. when I heard him speak in Britain in November.