Pros
It’s a solid company with a solid reputation. You'll gain experiences that will challenge you and bolster your resume. Generally speaking, work/life balance is good and management tends to allow a decent size leash on their staff. Many contract jobs allow for generous amounts of telecommute work. There are opportunities for advancement but you'll have to pursue those types of endeavors yourself, don't expect others to assist you. Pay and benefits are the unequivocal definition of mediocre, but that is nothing unordinary for many large companies.
Cons
You have to remember that much of Lockheed Martin's work is contract related. Although all full time employees are technically Lockheed Martin employees, please be aware that there is a tremendous difference between "corporate" employees and "contract" employees. Corporate staffing is a much smaller segment that is generally immune to the layoffs and financial fluctuations that create staffing issues. Corporate staffing manages and controls the contract work. If possible, corporate positions are much more desirable than contract positions. However contract employees make up the bulk of staffing at Lockheed Martin and, as expected, are very susceptible to the status of the contract. Employment is directly related to the health of the contract. When a contract is in it’s initial and execution stage, there is plenty of work and staffing naturally ramps up. But during the closing or re-bidding of the contract, resources can and will be reduced in order to meet requirements. I have personally witnessed staff members with 20 years of service receive 2 week notices with no warning. You must be aware of your role, and don’t take it personal. Resources (including contract employees) must constantly be controlled in order to maintain the contract. Generally speaking, most employment positions are safe along the life of the contract. However during re-negotiations or the closing period, all mid to lower level positions, understandably, will be subject to dispossession with tenure not currying much favor. Again, don’t take it personally, it’s an important financial decision corporate has to make to maintain the contract.