Slenderman in my experience was scary only the first time I played it, after that it was just focusing on getting the god damn notes and getting out of dodge. Much games did this even better than slenderman, but slenderman was just basically crowd hype, nothing more. They were good, but vague. If I was to give it a rating I would probably give it 5/10. Just my honest opinion, also I hate to spark a mean conversation but I saw Toad's post up there, one sharing his opinion and getting a down vote for that because he merely shared it is not nice.
Cheers.