This is debatable: "And his most acclaimed and influential works, which established him among the great composers of Romantic era music, were completed or drafted before he left Russia." Some would say his mature works are masterpieces whereas his early works are derivative (though finely composed).
Well, some might say that. I doubt that the vast majority of classical music listeners would say that. His reputation, popularity and influence were already well established by the time he got round to writing the Paganini Variations, the Symphonic Dances and his 3rd symphony.
