I did the Landmark Forum in New York about 12 years ago, so bear in mind everything that follows below may have changed in the subsequent years.
The introductory course runs over a weekend, and a weeknight (suppose it hasn't changed). There were about 100 people in my group. It is very intensive with 15 hour days. Alot of what they say makes good sense, and seemed to me to be a clever synthesis of principles from buddhism, judaism, christianity, psychoanalysis and savvy self help books. It is not religious in any way.
Some concepts are put forth in a quite obscure way that often seemed unecessarily complicated, but perhaps that is in order to make it sound new/fresh. They use some words and catch phrases in unusual ways which sounds like "forum-speak" and probably causes wariness when heard by "outsiders" who don't know the context.
As soon as the first weekend is over, you are encouraged (some would say pressured) to sign up for the advanced class. When that is done, they want you to do the Leadership class. And after that, there are lots of other classes available on specific subjects. I suppose the subsequent classes are meant to reinforce the concepts through repetition, so they want you to stay active. I stopped after the Leadership class. I had done all I wanted to do, and wanted a break. There was no pressure for me to continue further.
They don't advertise and are extremely wary of sceptics/critics who would level cult accusations - so they want graduates to spread the word to friends/family via word of mouth. Hence, the calls to friends, family etc to "clean up" past problems, and spread the word. (I'm sure being the recipient of one of those calls can be quite alarming when it comes out of the blue.) Beyond the introduction class, there is less expectation that you will "recruit" others. I paid for the classes I took, but was never asked to purchase anything else, and was never asked to contribute financially beyond the cost of the classes.
I don't think there is anything sinister about it, and wouldn't call it a cult. Somewhere in the very distant past, there was a tenuous link (via a previous organisation) to Est. People get worked up about the fact that you're asked not to wear a watch during the introductory classes. As there are scheduled breaks at regular intervals, you're also asked not to leave the main room to go to the bathroom while teaching is in session (they think your body could be distracting you from an uncomfortable/meaningful pyschological breakthrough). But, there's nothing sinister to it. I think those objections come from people who don't really want to do the course.
I found it to be interesting, thought-provoking, and valuable to a point. But I do question it's long term effectiveness unless you keep repeating/reinforcing the principles. Even now, 12 years later I'm having trouble recalling specifics so that I can write coherently about it here.