Brilliant movies that are difficult to watch

[Jason]

Are there any films that you adore but for one reason or another find difficulty in watching it?

Requiem For A Dream
A Clockwork Orange
Being John Malkovich (or any Charlie Kaufman penned film bar Eternal Sunshine)
Schindler's List (due to both the subject matter and the insane length of the film)

[Mike]

I didn't have a hard time watching a Clockwork Orange at all except for maybe a scene or two. Perhaps there is something wrong with me?

Length wise, Lawrence of Arabia. I loved it the first time I saw it, (I did watch it in two parts) but it's incredibly long and if you feel like watching it, you need a lot of time on your hands.