Yes, I'm very familiar with it.
I had a friend attending many years ago, and did a deep dive on the methodology they used there because I didn't trust it.
I can't seem to find it now, but I found an in-depth walkthrough of everything they do in one of these seminars. The first thing they do on Day 1 is to ask anyone who is only there because someone else pushed him to be there, and not because he is there of his own free choice, to stand up. Then once all those people are standing, they tell them to walk to the back of the room, get a refund, and leave. Of course this uses social pressure (nobody wants to be the one asshole walking down the aisle to claim a refund and slink out of there, tail between his legs) to force people to 'decide' on their own to stay, which creates buy-in. Everybody sits back down, and nobody leaves.
The rest of the seminar is like this. A bunch of NLP tricks and psychological mechanisms used to break down an individual's defenses, that use a meta dark pattern from NLP of breaking each person in the audience down, into sniveling, gibbering messes, then building them back up again, while anchoring the effect of the buildup phase to Landmark Forum itself.
The end result is that the participant comes out of it feeling like a new man -- and that Landmark is completely to thank for it!
Landmark then has each participant work his network to find new recruits. The recruits come to free sessions, which their friends guide them into signing up during, extolling the virtues of Landmark.
Everyone who attends will tell you his life has changed. He cannot tell you how it has changed, or any tangible benefits to it.
I will say it seems to help people in letting go of the past. So if you have trouble with that, it might be worthwhile.
It strikes me in many ways as analogous to a lot of other spiritual journey-esque events, like acid/mushroom trips, lengthy meditation retreats, rock-bottom life moments, and other things where you hit bottom and are remade. Usually those things are beneficial. So probably there is some benefit to Landmark -- it's an acid trip without the acid.
They do some really weird, acid-trip like things there, like using NLP and hypnosis to make you feel absolute terror that everyone around you is an evil being, then having you turn to look at your neighbor and realizing he is a friendly face, causing you profound relief (that reportedly leads to the whole audience laughing in relief, crying tears of joy, etc.).
But the people who go through it mostly seem to get way into it (with the odd grouser here and there).
I wasn't able to get my research to my friend in time -- he'd already completed the seminar by the time I told him about the methodology.
All he could do was smile about it, tell me it was wonderful, and try to get me to go.