The DLC for Outlast, 'Whistleblower', is more or less more the same sort of thing as the main game. And that'll either be a good or bad thing depending on who you are. They do switch up stuff, and they have a few new interesting stalkers, and a narrative that ties in well with the main game. But still, it for better or worse feels like an expansion of the game. If this is what you're looking for, you'll be very pleased. If you wanted something more different, this may not be it.
Something I noticed is that they tried to be a lot more radical with this one. And by radical, I mean they wanted to disturb the player and really make them feel uncomfortable. There's a specific section of the DLC that might really turn some people off, and this feels like the intended effect. And they deal with more radical ideas. It's hard to talk about this without spoiling elements of the DLC, but just know if you're offended, disgusted, or have an issue with nudity and mutilation, it is here in the forefront and trying to offend those with weaker appetites for the deranged.
You spend a little more time outside than you did in the main game, and most of the areas are new in the DLC (and those that you revisit have twists on them). There's also more on the overall story and outcome of the events that happened in the main game, so for those interested story-wise, it expands some on the universe and fallout from the events of the main game, as well as telling the personal story of somebody new who goes through the incident from beginning to end.
One thing I was curious about the DLC was how encounters would differ. One of the biggest flaws of the original game was that the encounters got very predictable after a while. Whistleblower does mix things up some, but honestly it was less different than I was hoping. There's no segments of 'Go to three points and exit' like the original game, and I noticed a bigger presence of back-tracking than I recalled there being in the original game. A few encounters were set-up to start at different points than you might expect, but they do feel like the same sort of thing. Though I did find the areas to be more designed to allow you to sneak around, the original game felt towards the end like it was throwing most of the same sort of things you'd already seen before during the last third of the game until the very end sequence, so I was hoping to see more mix-up here.
The new stalkers (of which there are two new main ones of) are both enjoyable. One makes a very distinct sound, which makes his encounters much more audio-based. The other is deliciously deranged, and charismatic. You also encounter a few of the original game's stalkers once or twice, but notably they seem to have chosen the stalkers that didn't appear much in the original either.
The DLC I would place at 2-4 hours long. It took me 3 & a half hours to complete it, but I got confused at a few segments of what I needed to do, and took some time to look at things.
It's well-made, and really does feel like an expansion. If that's what you want, more Outlast, this is it. And it's good. However, if Outlast dragged on you, then this is more of it. It's well-made, but not really all that different. There are some fun encounters and moments, however. But heed that it all gets more messed-up than the original game. I don't say that exaggeratedly, they obviously decided to take advantage of the fact that DLC is not rated, as if it was this goes into some ESRB AO-Rated territory.
A good piece of DLC that maybe isn't revolutionary or all that different from the main game, but is still well-made and worth experiencing if you happened to like the main game.