In his younger days, Rachmaninoff was not only a symphonic conductor, but conducted opera in Russia, and was in great demand there. He later shifted his emphasis mostly to composing as he had so much music on the drawing board. And finally, when he landed in the U.S., his first priority was making a livelihood, so his focus changed once again, this time to touring, and he did recitals and concerto appearances right up to the end of his life. He continued to compose during that period, but to a lesser extent, given his stringent time limitations, particular given his needs to expand his repertoire and to practice. (yes, Rachmaninoff practiced.) There is no question that he was an extraordinary conductor, composer and virtuoso artist. During his career touring, his only peer was Josef Hofmann.
As a composer Rachmaninoff wrote in just about all the forms, large and small--operas, songs, symphonies, concertos, choral works, chamber music, preludes, etudes, character pieces, duo-piano works, transcriptions, etc. And I believe it's fair to say that all of it is of high quality, which is why his music has endured so well. If I had to classify Rachmaninoff, I would call him both a Late Romantic and Neo-Romantic. Many of his 65 songs and the Piano Concerto No. 1, as examples, extend the Tchaikovsky tradition into the 20th Century. At the other end, I believe that the 4th Concerto, the Variations on a Theme by Corelli, and the Symphonic Dances attained a newer sound. That is not to suggest that Rachmaninoff was a revolutionary--clearly he was a conservative. Yet it seems that he was exploring and evolving toward a newer idiom.
If we did not have the music of Rachmaninoff, we would be much poorer for it. I think he was an extraordinary and masterful composer and a nearly peerless pianist.