Caligula is at the very least worth a watch so you can nod your head disapprovingly whenever it is mentioned. It is by almost any measure a terrible, terrible movie, but it's also not really boring, which is quite the feat for a movie that is 2.5 hours long. Most good movies can't even sustain that length.
Malcolm McDowell totally sells the movie. His scenery chewing would not have been out of place in one of the legitimately good sword and sandals movies. I think he's about 75% of the reason that Caligula is as watchable as it is. I mean, it's a terrible, terrible movie, but McDowell was a total professional about it, and he made the material as good as it possibly could have been.