Ventajas
The benefits are good. A lot of the people are good. Working for a big firm like LM does have pluses--they can get better health benefits than some firms, for example, because their size gives them leverage. In addition, the size of the firm makes it possible to have a long-term but varied career at LM. They are reluctant to lay people off a contract; someone who would be laid off is given first crack--and preference--at other jobs in the company. The management philosophy, as expressed by the maxims of Full Spectrum Leadership, is first rate.
Desventajas
Management is more concerned about the short-term bottom line than the longer-term needs of employees. Consequently, something like training can be given short shrift. Communication between employee and manager at the project level can be bad. This will likely vary by contract.