Maybe it helps to be familiar with the history, Art, and literature of the Ancient Rome because "Caligula" is surprisingly truthful adaptation of the chapter about Caligula in "The Lives Of The Twelve Caesars by C. Suetonius Tranquillus, the Roman Historian. If you read the chapter dedicated to Nero, you'll be even more shocked because Nero was always fascinated by his uncle Caligula (he was a son of Caligula's sister Agrippina who later became a wife of Claudius who adopted Nero and made him the heir for the title and the power of Roman Emperor). Anyway, Nero made Caligula his role model and managed to surpass his uncle's' notorious fame.
The movie is notoriously famous for the plentiful scenes of real sex, including incest, necrophilia, rape, and orgies. The movie also includes quite nasty and gruesome scenes of torture, executions, murders, and humiliations but all of the events have been documented in the historical documents that still exist.
I don't think of the movie as a masterpiece or even a good movie for all of its 2.5 hours. It actually reminds the life of real Caligula. In his childhood and youth, he was adored by Roman people and especially by the army and he was a promising young man. When he grew up as a heir to the cruel and suspicious Tiberius, he had to hide his feelings and go through many humiliations in order to survive. Shrewd Tiberius said about his adopted grandson that "never humankind knew the better slave and the worse ruler than Caligula" and that he was rearing "a viper for the Roman people and a Phaethon for the world."
When the young man finally received an access to the absolute power it had absolutely corrupted him. It is also known that soon after becoming head of Roman Empire, Caligula suffered an illness and as the result of it, he became incredibly nasty, cruel, and suspicious man who had indulged in the worst acts of debauchery, cruelty, and sadism. The movie follows this pattern. I still think that it is an interesting movie with very good actors. Not every day you can see porn with Helen Mirren, Peter O'Toole, Sir John Gielgud, and of course, Mr. Clockwork Orange himself, Malcolm McDowell.
