Ventajas
They really have created and propogated a great approach to management, where the managers are held responsible for maintaining a good work environment rather than a territorial "ownership" of the employees. And the work is challenging and interesting, moreso than other places that I've worked. A great place to work if you really want to do things. Compensation seems not only competitive but at the high end, from what I've seen. I'm sure that there are pockets in such a large organization where some of this doesn't hold, but I've been around (18 years experience at 9 different prime contractor sites) and I am very impressed with how ethical and how well-run Lockheed Martin is in general. Also their commitment to work-life balance - flexibility in work hours - is as real as it gets, and makes a big difference to me personally.
Desventajas
The TS&S work and location are a bit removed from the rest of the company. I hate to sound like a shill, but I really can't think of much negative to say about Lockheed Martin itself as opposed to negative aspects of particular jobs that don''t distinguish LM from other employers. Perhaps it is true that some of the workspaces - the buildings themselves and the work areas - seem cold and impersonal. At some places window offices are rare and most employees aren't near sunlight. If you're into your work that's not so much an issue, if not I guess it's a drag sometimes.