G. W. F. Hegel: “From the Kantian system and its completion I expect a revolution in Germany.”
Johann Holderlin: “Kant is the Moses of our nation.”
“Within a few years of the publication of his Critique of Pure Reason in 1781, Immanuel Kant (1724-1804) was recognized by his contemporaries as one of the seminal thinkers of modern times—indeed, as one of the great philosophers of all time.” (Allen Wood, “General Introduction” to Kant’s Religion and Rational Theology Cambridge, 1996, vii)
Schopenhauer on Kant’s moral philosophy: “I should liken Kant to a man at a ball, who all evening has been carrying on a love affair with a masked beauty in the vain hope of making a conquest, when at last she throws off her mask and reveals herself to be his wife.” (On the Basis of Morality)
Schiller, in a letter to Goethe: “There still remains something in Kant, as in Luther, that makes one think of a monk who has left his monastery, but been unable efface all traces of it.”
Moses Mendelssohn: Kant is “the all-destroyer.” (Quoted in Beck, Early German Philosophy: Kant and His Predecessors, 1969, 337).
Lewis White Beck: “Immanuel Kant was to put almost every fundamental concept of the Enlightenment in jeopardy.” (“German Philosophy,” p. 300).
Nietzsche: “Twice, when an honest, unequivocal, perfectly scientific way of thinking had been attained with tremendous fortitude and self-overcoming, the Germans managed to find devious paths to the old ‘ideal’—at bottom, formulas for a right to repudiate science, a right to lie. Leibniz and Kant—these two greatest brake shoes of intellectual integrity in Europe!” (Ecce Homo, 320)
Heinrich Heine: Our German philosophy is really but the dream of the French Revolution … Kant is our Robespierre.”
Heine’s 1834 prophecy: “Do not smile at my advice—the advice of a dreamer who warns you against Kantians, Fichteans, and naturphilosophers [e.g., Schelling]. Do not smile at the visionary who anticipates the same revolution in the realm of the visible as has taken place in the spiritual. Thought precedes action as lightning precedes thunder. German thunder is of true Germanic character; it is not very nimble, but rumbles along ponderously. Yet, it will come and when you hear a crashing such as never before has been heard in the world’s history, then you know that the German thunderbolt has fallen at last. At that uproar the eagles of the air will drop dead, and lions in the remotest deserts of Africa will hide in their royal dens. A play will be performed in Germany which will make the French Revolution look like an innocent idyll.”
Walter Kaufmann: “The only music Kant seemed to enjoy moderately was military marches.” (Nietzsche, Heidegger, and Buber: Discovering the Mind, Princeton 1980, p. 153)
George Santayana: “Kant had a private mysticism in reserve to raise upon the ruins of science and common-sense. Knowledge was to be removed to make way for faith … He wished to blast as insignificant, because ‘subjective,’ the whole structure of human intelligence, with all the lessons of experience and all the triumphs of human skill, and to attach absolute validity instead to certain echoes of his rigoristic religious education … Nature had been proved a figment of human imagination so that, once rid of all but a mock allegiance to her facts and laws, we might be free to invent any world we chose and believe it to be absolutely real and independent of nature.” (The Life of Reason, Volume 1, Chapter 4, “On Some Critics of This Discovery.”)
Karl Leonard Reinhold’s popular Kant lectures at Jena in the 1780s: “As a result of the enormously effective popularization of Kant by Reinhold, who lectured in the nearby university town of Jena, the area had become a breeding ground for scores of apostles of the Critical philosophy. When Reinhold left Jena in 1794, Fichte, Schelling, and Hegel took over in turn. They offered to improve on the ‘letter’ of Kant’s work in the name of its ‘spirit’.” (Karl Ameriks, The Cambridge Companion to German Idealism, Cambridge 2000, p. 2)
Klaus Christian Köhnke: “[I]n the 1870s Kant became the most frequently read of the classics in Germany’s universities” (The Rise of Neo-Kantianism, Cambridge University Press 1991, p. 8)
John Dewey: “A colleague of mine once suggested that old books, philosophic classics, be sent out by philosophic journals for review, to be criticized as if they had just issued from the press … . The two 
hundredth anniversary of the birth of Kant, falling in the month of April, suggests application of this method to the thinker who for the past seventy-five years supplied the bible of German thought.” (Characters and Events: Popular Essays in Social and Political Philosophy. Vol. I. Joseph Ratner, Ed. Henry Holt and Company: New York, 1929, 63)
John Passmore: “The Kantian revival is so widespread as scarcely to lend itself to illustration.” (Recent Philosophers, Open Court, 1985)
Christopher Janaway: “One feature uniting many kinds of recent philosophy is an increasing recognition that we are working within the legacy of Kant.” (“Introduction” to The Cambridge Companion to Schopenhauer, 1999, p. 3)
Related: