Absolutely haunting.
The first time I watched this I was probably twelve or thirteen years old. This remains one of the bleakest movie-watching experiences in memory, almost to the point I couldn't enjoy it... but I sat through (being a long-time fan of Mr. Christopher Lee) and ultimately, I'm glad I did.
As I said, it's bleak and dark while being totally hypnotic. Like driving past a car wreck with fresh blood smears on the road, you're horrified but you just can't look away. Lurking beneath the murky surface is a sense of doom. The ending is disturbing in a way I have not seen before or since.
I would not call this a horror film - at least, not in the traditional sense of the word. I'm not even sure if it can be probably categorized in one genre or another, and maybe it doesn't need to be. And let me clarify one thing: make no mistake, this is not a fun movie. It's not the thing you pop in when you're bored and looking to absentmindedly waste away an hour and forty minutes. The Wicker Man takes you by the throat with an icy grip and doesn't let go until long after the credits are over, if ever. It will remain with you for a very long time.
For a deeply disturbing and haunting experience, do yourself a favor and watch THE WICKER MAN.