Landmark is a step child of EST, a concoction of Werner Erhardt.
Are there powerful tools? you betcha. Do people get things out of it? They certainly feel that way. But Landmark, like LifeSpring (another EST stepchild) and EST itself, breaks your defenses/ego down without a greater context, and in the hands of often very egotistical "leaders."
These type of groups are also very much an intangible high without much substance.
I've know people who've been a part of the organization or been through the programs of all three. Save your money.
Here's some further reading, a study done on Lifespring:
Pathology as "Personal Growth": A Participant-Observation Study of Lifespring Training