The symphony is a piece of program music which tells the story of "an artist gifted with a lively imagination" who has "poisoned himself with opium" in the "depths of despair" because of "hopeless love." There are five movements, which was unconventional for a symphony at the time:
"Leonard Bernstein called this symphony the first musical expedition into psychedelia because of its hallucinatory and dream-like nature, and because history suggests Berlioz composed at least a portion of it under the influence of opium."
From http://en.wikipedia.org/wiki/Symphonie_Fantastique#Importance
I didn't know Leo felt the same way. Leo was a great conductor, but in my mind his music left much to be desired.