I don't know if Tchaikovsky really said it but here's a few others:

Ten composers on other composers:

"I have played over the music of that scoundrel Brahms. What a talentless bastard!" Tchaikovsky on Brahms

"His absurd cacophony will not be music even in the thirtieth century" Cui on Richard Strauss

"He makes me sad because he is really a cultured, agreeable man yet composes so very badly" Mendelssohn on Berlioz
"Obviously mad" Berlioz on Wagner

"Berlioz minus the melody" Auber on Wagner

"There was a gross error in the programme. Instead of 'Symphony' they should have printed 'Cacophony'" Arensky on Scriabin

"I found the Second Symphony to be vulgar, self-indulgent, and provincial beyond all description" Virgil Thomson on Sibelius

"The [Ninth Symphony's] fourth movement is so ugly, in such bad taste, and the conception of Schiller's Ode so cheap, that I cannot understand how such a genius as Beethoven could write it down" Spohr on Beethoven

"The Dante Symphony is hell, and the tone poems survive only by constantly renewed neglect" Stravinsky on Liszt

"Prélude à l'après-midi d'un faune has pretty sonority, but
one does not find in it the least musical idea" Saint-Saëns on Debussy

And I thought it was only Salieri's thoughts about Mozart that had survived (or was that made up for the play?)

http://www.classiccd.co.uk/fun/Lists/List26.html