Decent watch at best, probably won't watch again, and can't really recommend.
I'm a big fan of Joey King, but she wasn't really in a positive to save the movie.
The production value is obviously there, the effects are even good, and there are several intense moments that should set the atmosphere, but I just couldn't bring myself to care.
The writing has to be to blame. It's 4-5 shallow, empty characters with just enough flavor to tell them apart, and I just can't care about them.
The movie focuses on the subtly of it all, the growing madness that Slender Man instills in his victims without properly building the tension or showing the characters slowly losing it, which would have given good actors the ability to save it. Instead they use a lot of "it happened offscreen" or sudden cuts to show a lose of time, and we get a character suddenly looking confused instead of taking the audience on a journey with the character.
Maybe it's not the writing, and maybe the editor just didn't know what they were doing? "What are all these long dramatic series in here? This is so boring! cut Eh, they'll get the point."
With all the options out there, I just don't think this is worth your time. I could tell you to just watch the 3rd act, and you'd probably get as much out of this movie as you're going to get from the beginning.