Both of these are worth a read. A couple of excerpts:
The Roman Missal promulgated by Paul VI is the ordinary expression of the ‘Lex orandi’ (Law of prayer) of the Catholic Church of the Latin rite. Nonetheless, the Roman Missal promulgated by St. Pius V and reissued by Bl. John XXIII is to be considered as an extraordinary expression of that same ‘Lex orandi,’ and must be given due honour for its venerable and ancient usage. These two expressions of the Church’s Lex orandi will in no any way lead to a division in the Church’s ‘Lex credendi’ (Law of belief). They are, in fact two usages of the one Roman rite.
It is, therefore, permissible to celebrate the Sacrifice of the Mass following the typical edition of the Roman Missal promulgated by Bl. John XXIII in 1962 and never abrogated, as an extraordinary form of the Liturgy of the Church.
(Pope Benedict XVI, Summorum Pontificum, Article 1, July 7, 2007)
Many people who clearly accepted the binding character of the Second Vatican Council, and were faithful to the Pope and the Bishops, nonetheless also desired to recover the form of the sacred liturgy that was dear to them. This occurred above all because in many places celebrations were not faithful to the prescriptions of the new Missal, but the latter actually was understood as authorizing or even requiring creativity, which frequently led to deformations of the liturgy which were hard to bear. I am speaking from experience, since I too lived through that period with all its hopes and its confusion. And I have seen how arbitrary deformations of the liturgy caused deep pain to individuals totally rooted in the faith of the Church.
(Pope Benedict XVI, Explanatory Letter on Summorum Pontificum, July 7, 2007)