This is one of British cinema's best ever films and it is very powerful. A lot of people worship it and it now enjoys cult status, but when it was first released it was not really a success, mainly because of the poor marketing and support. But it's reputation grew over time. I guess that you just can't keep a good film down!
I think the label "horror film" is misleading, and I suspect that it is the reason why some of the people who watch it come away perplexed or, in some cases, disappointed. In fact, it isn't really a true horror film, although there are undoubtably some horrific moments in it. But it also contains the aspects of a mystery story, a drama, even a musical! It's a strange hotch potch that defies categorisation, and in this sense, is somewhat unique. Not many films are brave enough to break from established convention. This one did and it will enjoy classical status one day, if it doesn't already.