I'd been a fan of Slender Man for some time before Slender came out. Out of boredom and wanting to find something legitimately creepy, I stumbled across a link to the Something Awful post where the character was conceived. This lead me to Marble Hornets, which was midway through season 1 at the time. There was just something unsettling about this... thing, this tall, tentacled, suit-wearing faceless man, that made reading about and watching him (it?) so fascinating.
When I heard there was a video game starring Slendy as the antagonist, I knew I had to go for it. Fortunately, I was not disappointed. Though a bit rough, it was miles beyond what I anticipated. Even when I knew he was coming, it still startled the hell out of me when he popped on the screen for the "kill".
Literally the only thing I have against Slender is that, like everything good, it spawned a massive amount of crap copies and imitators. The sheer mountain of games that follow the exact same formula, however, has only slightly dulled the amazing experience of Slender, and if you ignore the clones, Slender remains pristine.