Ok so I just played the Outlast DLC myself, and I'll share some of my thoughts on it!
In this DLC you play as Waylon Park who is the employee who leaked the info to Miles from the original game. He is a better character than Miles in the fact that he simply HAS a character. In seriousness, it's not a particularly engaging, or compelling one, but it's still nice to have some story to your character to give that extra bit of oomfpt to making the game as a whole engaging.
The DLC is mainly split into two "sections" which each have their own "villain", let's just call it that.
The first half is in my opinion extremely meh. Mostly it is, because it is constantly throwing stuff at you, and does not work to get its atmosphere up at all. Also, the main gimmick of the villain didn't do too much for me personally either. In this area I mainly found myself mildly bored while going through the levels, and not really caring about anything. I was not immersed in the slightest in this part. This can very much be personal preference though.
The second half however...ohohoho the second half in my opinion reverses all the problems the first half had around. It builds atmosphere, hypes up the big bad with conversations beforehand, and his entrance is just bone-chilling. Also his main "gimmick", and speech patterns were enough to raise hairs on the back of my neck. His part was easily the most scared I had ever been in Outlast.
I only hope that he did not rely so much on gorey sections to try, and get his "horror" across. There is a TON of gore in this DLC to the point that I had no doubts not calling this "decoration", and just "torture porn" in some areas. It's nice effective decoration in most areas, but in a couple it actually takes away from the scare factor of the game.
Another big issue I had with the game is the level design. The most glaring problem is that you can often cannot figure out where to go. In safe zones this means you'll be running around a lot looking for that one ledge to climb, and in enemy zones it means you'll be dying a lot because you couldn't find the place to run away to. It gets very frustrating after a bit.
All in all though, I'd say the DLC is generally "good", and most definitely worth the pickup price.