First, he restored prayer to the world of Yeshiva spirituality. Davening in Volozhin was notable for the brevity with which it was marked. There, the Study of Torah reigned supreme and the time allotted there to was maximized. The Rav, whose all encompassing involvement in Talmud Torah was no less intense than that of his forebears in Volozhin and Brisk, made extraordinary efforts to sensitize his disciples to the text of the siddur and the riches it contains. By so doing, he balanced out the perennial tension between Prayer and Talmud Torah as competing spiritual activities, a tension that marks Judaism from the beginning.
The Rav also made prayer both accessible and desirable for the intelligent modern whose daily life mires him in quotidian trifles, and renders him obtuse to Eternity, and to his Creator. He did this in his inimitable way by harnessing the totality of Torah and Western Culture to explicate both Halakhic discussions concerning the commandment to pray, and the text of the prayers themselves (and Worship of the Heart is but a foretaste of a much larger discussion).
So, the publication of this Siddur should be greeted with enthusiasm and gratitude to the many people involved in its production.
I am concerned that in all of the blessed publication of this material, תורת הרב הכלכך קרובה ללבי, a central part of his teachings will get lost: study.
The Rav did not write a commentary to the liturgy. He studied, very closely and creatively, the mahzor and the siddur, piyyut and tefillah. It was in the interaction between mind and heart, in the stretching of the mind and the invocation of interpretive creativity that the Rav was in his metier. He demanded not only results but process, from both himself and his students. I do not believe he was interested in a Soloveitchik canon for Divrei Torah and ווארטלטך, but rather to show the way to ever deeper understanding of the words of the liturgy, which will resound in the heart of the Jew as he/she undertakes the challenge of prayer.
So, in the end, whether this prayerbook (as with other collections of the Rav's interpretations) is truly Massoret ha-Rav (ie in the tradition of Rav Soloveitchik) will be determined not by its publication but by how it is used.