Pros
I am a scientist who started with SAIC in the mid-90's, world through he "going public", multiple CEO's, and the transition to Leidos. I have worked my way up from someone green to a rather senior level where I am a primary decision maker on multiple contracts. My salary has increased by 4x over the 23 years -- growth exists. I am surrounded by really smart people who love solving problems -- hard problems that are of critical importance to the nation. Salary is pretty good (I think, not looking around), benefits are Meh. But that is the case throughout the industry. For me, the biggest plus is, most mornings, I am excited to go to work and I find the work engaging. I get the tools I need to do the job by one asking once. One other thing, there is a lot of personal flexibility. In my group, as long as the work gets done (and 80 hours in two weeks are billed), no one cares which hours are worked.
Cons
First of all, we are a contracting company. If we do not win new work or recompense, we are laid off, and are on our own to find a new position. The company offers some limited help. No severance, though. But, if you are good, and you network within the company, you can have continuity of employment. In fact, that is what usually happens. Getting a layoff notice, while incredibly stressful, is not considered an embarrassment. But, it does mean that you need to be 100% aware of situations. Let me be clear, though, this is not unique to Leidos. I have friends in other companies that are in the same boat. Basically, if there is no work, there is no job. Other big negatives is, over the last several years, bonuses have been lacking/absorbed by the higher ups.