So I watched it.
Went down basically how I imagined. I saw it, got freaked out for a while, then I moved on.
It was intriguing to see the mythos come to life, and I honestly think they handled it pretty well. However, my girlfriend and I both agreed their approach to explaining Slender Man and presenting the movie's "message" was kinda hamfisted.
I did really enjoy seeing Slender Man on screen whenever he showed up (a total of probably 4 or 5 times), and there was some really stunning imagery, especially in the penultimate scene.
The characters are forgettable, but they were well-acted. They felt like real teenage girls, which is good, but none of them really have any defining traits and characteristics, and I didn't even know the main character's name until the very end of the movie. Of course (MOVIE SPOILER?) it didn't really matter because they all die anyways.
My thoughts can be summed up by basically saying this movie did everything pretty well except for story and character. It depends on who you are if you think going to see it is worth it.
I think it's a decent watch when it leaves theaters, and while I'd be tempted to see it again for the images alone, I'd be hard pressed to do so considering my existing unease with the character.
Also, if it were up to me, I'd have gone with more images of Slender Man just standing and watching. That's one of the most iconic things he does in his mythos, and it's shamefully only used about twice, and only one of those times is it in broad daylight where you can see him in detail. Instead of doing that and going for the creep factor that Slender is very good at evoking, they went with some pretty cheap jump scares. That being said, I did find the movie's connection of trees and Slender Man to be pretty cool and unique, especially in the scene I mentioned earlier.