LIGHT SPOILERS BELOW.
I finished this a week ago and I haven't stopped thinking about it. What a well-crafted story. As other commenters noted, the pacing is immaculate and each revelation about our characters and the situation they're in is perfect right up until the end where we're asked to make one last "leap" in logic, which I think was perfectly fair even if it stumped me for an hour plus. The biggest revelation for me was not even that final puzzle, it was the one that requires us to make the same deduction about a character we know existed but none of the characters in our story remember. Now THAT'S a great puzzle -- made me gasp out loud when I reread those early logs as I realized what this meant for the story. Lastly, I think the framing device is perfect and adds a deep sense of mystery and eventually horror as we realize what happened to all these people, the first investigators and now, presumably, us...