One of the only Slender games I've played, and overall it was pretty fun. I liked the atmosphere and visual design a lot.
I did encounter an issue where I couldn't reload my flashlight's battery. I kept pressing R while holding my flashlight, which was specified as reload in controls, but the flashlight simply wouldn't change batteries.
One other personal criticism/suggestion I have is regarding the sound that Slenderman makes when attacking. For me, when I heard the sound he made, I laughed because I knew instantly where it was from and it broke the immersion. In general the sound doesn't really fit the character in my opinion, but it's just something I noticed.
Aside from that though, I enjoyed Part One, and can't wait to check out when the game's updated.