Ventajas
Been with LM for nearly a decade. I've had the privilege of having great supervisors who genuinely were concerned about my professional growth, etc. The pay is similar to any of the large DoD contracting firms Benefits are great, however, they are going through some substantial changes to better align with current economic strategies within the US. I'm sure that will continue with the current CEO in place who knows how to pinch a penny and turn it into a profit.
Desventajas
Can be difficult to find training for specific career fields given the nature of contract work. Some of a the divisions will not pay for training out of overhead and unless the contract specifically states paid training, you can get yourself into a dead-end situation. Promotions can be difficult as well depending on your supervisor and what contract you support; if the contract was bid on certain salary grades you could find yourself in a bad spot.