Update: the full text of Packer's article is available by kind permission of Uccf at the Reformation21 blog.
NB: news, the magazine for supporters produced by uccf:thechristianunions has a three page feature article by Jim Packer on "Penal Substitution Revisited." Unless it appears on the uccf website I suggest you go here to obtain a copy and to receive regular updates on student work. The full text of Packer's classic lecture What did the Cross Achieve? the Logic of Penal Substitution (Tyndale Biblical Theology Lecture, 1973) is available online here.
Here are a few quotes from the uccf article to give you a taster:
Throughout my 63 years as an evangelical believer, the penal substitutionary understanding of the cross has been a flashpoint of controversy and division among Protestants. It was so before my time, in the bitter parting of ways between conservative and liberal evangelicals in the Church of England, and between the Inter-Varsity Fellowship (now UCCF) and Student Christian Movement (SCM) in the student world. It remains so, as liberalism keeps reinventing itself and luring evangelicals away from their heritage.
Since one's belief about the atonement is bound up with one's belief about the character of God, the terms of the gospel and the Christian's inner life, the intensity of the debate is understandable. If one view is right, others are more or less wrong, and the definition of Christianity itself comes to be at stake.
As I grow old, I want to tell everyone who will listen: 'I am so thankful for the penal substitutionary death of Christ. No hope without it.'
It was with his own will and his own love mirroring the Father's, therefore, that he took the place of human sinners exposed to divine judgment and laid down his life as a sacrifice for them, entering fully into the state and experience of death that was due to them.
Since all this [the work of the Triune God in salvation] was planned by the holy Three in their eternal solidarity of mutual love, and since the Father's central purpose in it all was and is to glorify and exalt the Son as Saviour and Head of a new humanity, smartypants notions like 'divine child abuse,' as a comment on the cross, are supremely silly, and as irrelevant and wrong as they could possibly be.
Both testaments...confirm that judicial retribution from God awaits those whose sins are not covered by a substitutionary sacrifice.
I, having surveyed the penal substitutionary sacrifice of Christ afresh, now reaffirm that here I rest my hope. So, I believe, will all truly faithful believers.