I haven't watched any Markiplier videos in years, and knew nothing about this before its sudden release when it was everywhere. I decided to give it a whirl, and it's worth a watch I'd say.
For the positives, I think at a high level it's an intriguing premise. What would you do if 99% of the population died in their sleep, and you had to try and figure out what to do without being able to sleep yourself? There's some creepy segments and imagery, and honestly Mark's not a terrible actor. He's not going to be winning an Emmy or anything but I thought he did a decent enough job for someone who got famous by playing games and acting like a goofball on YouTube. It's also very quick, 6 twenty minute episodes, and they move along pretty briskly. It could easily be knocked out in an evening.
For the cons, the main one is probably that it's a little art-housy. Everything that happens does get some kind of explanation, but I could understand people being frustrated at how light some of the explanations are. There's also some Hollywood Logic on display, of people being able to do things or act in a certain way that real people wouldn't be able to do. The general script and acting also aren't amazing, although both were serviceable enough. Also, the last halfish of the final episode is basically just a big exposition dump, which is always annoying.
Really though, I think it's a small enough investment that it's worth giving a shot. It's basically the length of a movie, and there's enough creepy imagery to make it worth a watch.