I'll start off by pointing out that Slender had some major post-production hurdles to jump. Due to ownership rights of the Slenderman character, the film-makers apparently had to edit out a lot of references to, and on-screen representations of the familiar character. I write this review with that in mind.
In all honesty, I think the film-makers had some good approaches and nice ideas. There are several scenes where the actors are clearly off-script and just ad-libing their lines and actions. These scenes really stand out as believable and memorable, and hark back to the "guerilla filming" approach of The Blair Witch Project.
Sadly, and unlike The Blair Witch Project, this film has a script that is both badly thought-out, and seems to be too rigidly adhered to for the style of film the creators seemed to be trying to make.
There are frequent moments where its hard to believe that any single person - let alone and entire film crew - could go along with what they have planned. The final section, in particular, sees one character behave like an utter sociopath, and the surrounding characters initially object before quickly deciding to join in, despite all of their moral and logical objections, with no real reason for doing so.
There are several scenes where Slender Man was clearly pictures or mentioned, but was then edited out. To be fair. These edits are presented as visual or audio glitches which fit in with the story and myhtis of the character, and are largely well-handled. I've watched this film twice and it was only after reading about the legal issues before my second viewing that I could spit that these edits were censoring things out. Some of these scenes suggest there were subtle hints of Slenderman in the background but they could just have easily been cheap jumo-scares in some scenes. Given the editing and censorship, it's hard to tell what these scenes would have looked like if presented as originally intended.
Overall though, and even making allowance for the troubled post-production, Slender just doesn't hit the marks it aims for.
Far too much of the film is taken up with letting us get to know the main characters, with very little left for plot profession. The horror aspect, which is what a film like this is really all about, only really comes into play in the final 15-20 minutes and, even then, is largely a montage of angsty characters fretting over where other characters have gone.
The characters themselves are hard to pin down. In some scenes they seem genuine and relatable, but in others act as cheap movie clichés which are utterly unbelievable.
It's a shame. I thing the creators has good intentions for this film and approached some things in novel and effective ways. But, ultimately, these few imaginative sparks were obscured my poor writing and mediocre execution. Forced censorship of any direct Slender Man references wasn't so much the nail in the coffin as patting down the earth on the grave.
I would be interested to see what the film-makers do in future - there's potential there - but I don't think Slender demonstrates that potential anywhere clearly enough.